Flaxseed

Flaxseed (Linum usitatissimum L.): A Comprehensive Reference

1. Identity and Botanical Classification

Flaxseed is the common name of Linum usitatissimum L., also known as common flax when grown for the fiber extracted from its stem, and as linseed or oilseed flax when cultivated for the oil extracted from its seeds. It belongs to the genus Linum L. and the family Linaceae. Flaxseed is the seed from the flax plant, an annual herb. It thrives in deep, moist soils rich in sand, silt, and clay. The species is native to the region extending from the eastern Mediterranean, through Western Asia and the Middle East, to India.

The Latin name of flaxseed (Linum usitatissimum L.) means "very useful," and it has two basic varieties: brown and yellow or golden (also known as golden linseeds). Mature flaxseed is oblong and flattened, comprising an embryo with two cotyledons surrounded by a thin endosperm and a smooth, often shiny yellow to dark brown seed coat (hull).

Common Forms and Preparations

Commercially, flaxseed is available in three different food forms, including whole flaxseed, milled flaxseed, and flaxseed oil. Flaxseed has gained worldwide recognition as a health food because of its abundance in diverse nutrients and bioactive compounds such as oil, fatty acids, proteins, peptides, fiber, lignans, carbohydrates, mucilage, and micronutrients. These constituents attribute a multitude of beneficial properties to flaxseed that make its use possible in various applications, such as nutraceuticals, food products, cosmetics, and biomaterials.

The bioavailability of alpha-linolenic acid (ALA) is dependent on the type of flax ingested — ALA has greater bioavailability in oil than in milled seed, and has greater bioavailability in milled seed than in whole seed. Flaxseed ingestion over a one-month period resulted in significant increases in plasma ALA levels in the flaxseed oil and the milled flaxseed supplemented groups, with flaxseed oil producing significantly higher ALA levels than the milled flaxseed group. Subjects supplemented with whole flaxseed did not achieve a significant increase in plasma ALA levels. Crushing and milling of flaxseed substantially improve the bioavailability of enterolignans, likely due to the improved accessibility of colon bacteria to crushed and ground flaxseed.

Unlike whole flaxseeds, flaxseed oil lacks fiber but provides a more concentrated and bioavailable source of alpha-linolenic acid (ALA), the plant-based omega-3 fatty acid. Standardized lignan extracts, typically concentrated from the seed hull, are also commercially available as dietary supplements.

2. Historical and Traditional Use

Flaxseed (Linum usitatissimum L.) is one of the oldest cultivated crops and continues to be widely grown for oil, fiber, and food. Flax is one of the oldest cultivated plants in human history, dating back to Neolithic times — around 6,000 years ago. In ancient Egypt, the linen used to wrap mummies was made entirely from flax fibres, a testament to its strength, purity, and importance in early civilization.

Traditionally, flaxseed (linseed) has been utilized for textile production, varnishes, and medicinal application, including the treatment of respiratory, digestive, and dermatological disorders. The plant was first cultivated as early as 3000 BC in Mesopotamia, where it was believed to possess significant health benefits. Through the course of history, the flax plant enjoyed widespread use in ancient Greek cuisine and in ancient Roman medicine. Its historical medicinal uses include being used as a laxative as well as an expectorant responsible for soothing irritated tissue, controlling coughs, and even relieving pain.

Pliny the Elder recorded flaxseed used in the rations of Roman soldiers, added to multi-grain porridge or gruel to give them increased endurance. Emperor Charlemagne decreed that the people of France were to grow and consume flaxseed for their health. Nicholas Culpeper, the herbalist, mentioned "cold-drawn" linseed (flaxseed) oil as a treatment for many conditions including inflammation, tumours, chest infections, and abscesses.

The use of flax spread until it reached ancient populations of the North, located in England, Denmark, and ancient Scandinavian countries. The use of flax has survived to the present day, and because of its rich history through so many different cultures, it was also charged with strong allegory and depicted in paintings and illustrations for millennia.

The Slavic tribes were the first to begin cultivating flax in Eastern Europe, having brought it from Greece. It was used to make fishing nets, ropes, sailcloth, and linseed oil. By the 10th and 11th centuries AD, flax was grown extensively in Russia.

In South Asia, Ayurvedic tradition recognized flax for balancing the body's systems and supporting overall health. In China and India, flax seeds were valued for their healing properties. Traditional Chinese medicine uses flax seeds to aid digestion and promote overall health, while in India, herbal medicine specialists value them for their ability to balance the body's energy. Even today, flax seeds remain essential to Indian diets, often added to flatbreads and herbal remedies.

3. Chemical Composition and Key Constituents

Macronutrient Profile

Analysis of brown Canadian flaxseed conducted by the Canadian Grain Commission showed the average composition of commercial seed was 41% fat, 20% protein, 28% total dietary fiber, 7.7% moisture, and 3.4% ash. Minor components included: cyanogenic glycosides, phytic acid, phenolics, trypsin inhibitor, linatine, lignans (phytoestrogens), minerals, vitamins, cadmium, selenium, and cyclolinopeptides.

Alpha-Linolenic Acid (ALA) — Omega-3 Fatty Acid

Flax is one of the richest plant sources of essential n-3 and n-6 fatty acids, primarily alpha-linolenic acid (ALA, C18:3 n-3) and linoleic acid (LA, C18:2 n-6). Alpha-linolenic acid (ALA) is an essential polyunsaturated fatty acid with an omega carbon at position three, which cannot be synthesized by the human body itself. Flaxseed oil is copious in unsaturated fatty acids (87.8 to 89.8%) in contrast with the small sum of saturated fatty acids.

ALA from flaxseed can be elongated and desaturated in the body to produce the longer-chain omega-3 fatty acids EPA and DHA, but this conversion process occurs primarily in the liver but is notably inefficient — research suggests that approximately 5–10% of ALA converts to EPA, and less than 0.5–5% converts to DHA. These rates are approximate and vary considerably between individuals, with women typically converting more efficiently than men. Genetics and overall dietary composition also influence conversion efficiency.

Notably, no significant increase was detected in plasma eicosapentaenoic acid (EPA) or docosahexaenoic acid (DHA) levels in any of the flax-fed groups in one human bioavailability study, underscoring the limited practical conversion of ALA to the longer-chain forms in most individuals.

Lignans — Secoisolariciresinol Diglucoside (SDG)

Flaxseed is the richest source of providing the lignan precursor secoisolariciresinol diglucoside (SDG). Flaxseed contains levels of phytoestrogens which are generally 75–800 times greater than any other plant food.

Flaxseed is the richest source of the lignan secoisolariciresinol diglucoside (SDG). After ingestion, SDG is converted to secoisolariciresinol, which is further metabolised to the mammalian lignans enterodiol and enterolactone. A growing body of evidence suggests that SDG metabolites may provide health benefits due to their weak oestrogenic or anti-oestrogenic effects, antioxidant activity, ability to induce phase 2 proteins and/or inhibit the activity of certain enzymes, or by mechanisms yet unidentified.

The flaxseed lignan SDG is biotransformed by bacteria in the gastrointestinal tract upon oral intake. Due to their lipophilicity, the aglycones and mammalian lignans may cross biological membranes via passive diffusion. Bioactivity of the lignans also requires their adequate systemic exposure following oral consumption. Systemic exposure of the lignans is generally quite low due to their limited oral bioavailability.

Dietary Fiber and Mucilage

Flaxseed gum constitutes almost 8–10% of the whole seed weight. It attributes unique functional properties to foods owing to its unique polysaccharide composition, such as improving viscosity, emulsifying ability, rheology, and foaming ability. The soluble fiber fraction, or mucilage, is responsible for many of the seed's effects on gut motility, glycemic response, and cholesterol levels.

Protein and Peptides

Orbitides (cyclolinopeptides) in flaxseed mostly consist of 8–10 amino acids. The structure of one famous Cyclolinopeptide-A is Pro-Pro-Phe-Phe-Leu-Ile-Ile-Leu-Val. These compounds have been found to possess multiple beneficial traits, such as being immunosuppressive, anti-malarial, anti-tumor, and a protectant against bone degeneration.

Phenolic Compounds

Phenolic compounds of flaxseed are mainly comprising of two classes, which are phenolic acids and lignans. The phenolic acids are mainly ferulic acid, coumaric acid, chlorogenic acid, vanillic acid, and p-hydroxybenzoic acid. Phenolic acids increase cells' antioxidant and anti-inflammatory defenses against damage in cell and animal studies. Emerging evidence in animal studies suggests they may also improve glucose metabolism, decrease insulin resistance, and alter the gut microbiota, creating an environment in the body less likely to support cancer.

Anti-nutritional and Potentially Toxic Components

The major anti-nutritive compounds in flaxseed are linatine, phytic acids, protease inhibitors, and cyanogenic glycosides (linustatin, neolinustatin, linamarin, and lotaustralin). Phytic acid interferes with the absorption of minerals such as calcium, zinc, magnesium, copper, and iron, whereas, upon hydrolysis, cyanogenic glycosides release hydrogen cyanide, a respiratory inhibitor, which in turn is converted to thiocyanates. Thiocyanates impede iodine uptake by the thyroid gland and long-term exposure can exacerbate iodine-deficiency disorders such as goiter and cretinism.

4. Mechanisms of Action

Lipid Modulation

Animal and human studies have suggested that SDG and its metabolites mediate serum total cholesterol, low-density lipoprotein, total cholesterol and high-density lipoprotein ratio, which leads to less androgenic complication and antioxidative prevention. A series of research studies indicates that regular flaxseed with alpha-linolenic acid and flax lignan polymer (containing 34–38% SDG, 10–11% 3-hydroxy-3-methylglutaric acid, and 15–21% cinnamic acids) as potential bioactive components, or purified SDG in equimolar concentration, have similar antiatherogenic effects.

Antioxidant and Anti-inflammatory Activity

Various studies have shown that SDG offers several health benefits, including protective effects against cardiovascular diseases, diabetes, cancer, and mental stress. These health benefits have been attributed to the antioxidant properties of SDG. The ex vivo antioxidant activities of SDG, enterolactone, and enterodiol were shown by specifically inhibiting linoleic acid lipid peroxidation, indicating direct hydroxyl radical scavenging activity. In addition, due to their ability to inhibit platelet activating factor (PAF), lignans may exert antioxidant activity by inhibiting reactive oxygen species (ROS) production by white blood cells.

Phytoestrogenic and Hormonal Modulation

Although flaxseed possesses beneficial fatty acid composition and high fiber content, the phytoestrogenic, anticarcinogenic, and antiatherogenic effects have been attributed to its lignan content. The mammalian lignans enterodiol and enterolactone function as weak phytoestrogens; when lignans reach the digestive tract, bacteria in the gut microbiome get to work on them to form by-products called enterodiol and enterolactone. These metabolites have a similar structure to oestrogen, so they can bind to oestrogen receptors in the body and exert a gentle modulating effect on female hormones.

Adipogenesis and Metabolic Regulation

In experimental animal trials, SDG significantly reduced high-fat diet-induced visceral and liver fat accumulation, hyperlipidemia, hypercholesterolemia, hyperinsulinemia, and hyperleptinemia. The mechanism proposed for these actions was the regulation of adipogenesis-related gene expression through an increase in peroxisome proliferator-activated receptor-gamma-mediated DNA binding activity induced by flaxseed lignans.

Anti-cancer Mechanisms

Recent scientific research has increasingly confirmed flaxseed's anti-cancer properties, particularly its effectiveness in preventing hormone-related cancers such as breast and prostate cancer, and its ability to inhibit tumor growth. These effects are largely attributed to its rich content of omega-3 fatty acids, lignans, and polyphenols, which help reduce oxidative stress, inflammation, and the risk of chronic diseases. One of the lignans present in flaxseed, secoisolariciresinol diglucoside (SDG), has been identified as a strong inhibitor of the 5α-reductase enzyme, which in some cases may contribute to the development of prostate cancer.

5. Scientific Evidence by Health Area

5.1 Cardiovascular Health — Lipid Profiles

Owing to promising results in preclinical models, many clinical trials have been performed to determine the outcomes of flaxseed intervention (whole flaxseed, flaxseed oil, or lignans) on various cardiometabolic risk factors, particularly blood lipids. However, the findings from most of the previous clinical trials were inconsistent, and the discrepancies could be attributed to small sample size, insufficient study duration, variation in study designs, and diversity of the test product.

In a double-blinded, randomized, controlled clinical trial: sixty-two men and post-menopausal women with pre-study LDL-C between 130 and 200 mg/dL were randomized to 40 g/day of ground flaxseed-containing baked products or matching wheat bran products for 10 weeks while following a low-fat, low-cholesterol diet. Fasting lipoproteins, measures of insulin resistance, inflammation, oxidative stress, and safety were assessed at 0, 5, and 10 weeks. Flaxseed was well-tolerated and increased serum levels of ALA. Compared to wheat, flaxseed significantly reduced LDL-C at 5 weeks (−13%, p < 0.005), but not at 10 weeks (−7%, p = 0.07). The authors concluded that ground flaxseed has a modest but short-lived LDL-C lowering effect, yet reduces Lp(a) and improves insulin sensitivity in hyperlipidemic adults. The HDL-C lowering effect of flaxseed in men warrants additional study.

Regarding lipoprotein(a): in a recent double-blinded, randomized, controlled clinical trial, Bloedon et al. demonstrated that 40 g/day of ground flaxseed reduced Lp(a) by 14% after 10 weeks of supplementation. Similarly, Arjmandi et al. reported that 38 g/day of whole flaxseed lowered Lp(a) by 7.4% in postmenopausal women after six weeks of treatment.

A randomized double-blind placebo-controlled trial in 80 hyperlipidemic and hypertensive patients: this trial was performed on men and women between 20 and 60 years old. Participants were recruited from Imam Khomeini hospital clinics of Shiraz University of Medical Sciences in 2017, and randomly allocated to flaxseed powder group and placebo group. The intervention group received 36 g of flaxseed sachet (n = 40), and the control group received 12 g placebo sachet (n = 40) for 8 weeks. The researchers found significant reductions in anthropometric indices (waist circumference and waist-to-hip ratio) and lipid profiles (triglycerides [P = 0.015], total cholesterol [P = 0.018], and low-density lipoprotein [P < 0.001]) within the flaxseed group in comparison with placebo.

Evidence strength: The lipid-lowering effect of flaxseed is supported by multiple RCTs and meta-analyses, but effect sizes are generally modest and heterogeneity across studies is high. Results for LDL-C are more consistent than for triglycerides or HDL-C. Whole-seed studies generally show weaker effects than milled seed studies.

5.2 Cardiovascular Health — Blood Pressure

A 2025 systematic review and meta-analysis examined blood pressure outcomes: PubMed, Scopus, Web of Science, and Cochrane Central Library databases were searched, and eighteen RCTs were included in the present systematic review and meta-analysis. Pooled analysis suggested that flaxseed supplementation can reduce systolic BP (WMD: −4.75 mmHg, 95% CI: −7.05 to −2.44, P ≤ 0.001) and diastolic BP (WMD: −3.09 mmHg, 95% CI: −4.37 to −1.81, P ≤ 0.001). Given the significant heterogeneity (I² = 93.6% for SBP; 91.2% for DBP), it is crucial to interpret the current results with careful consideration. Further high-quality RCTs are required to better assess the causal relationships.

Evidence strength: Statistically significant blood pressure reductions have been observed in meta-analysis, but the very high heterogeneity across trials limits the strength of conclusions. Effect sizes are clinically modest.

5.3 Glycemic Control and Type 2 Diabetes

Flaxseed supplementation may improve glycemic control and insulin sensitivity in healthy participants and patients with type 2 diabetes.

In a randomized controlled trial: 53 constipated patients with T2D received either 10 g of flaxseed pre-mixed in cookies twice per day or placebo cookies for 12 weeks. Constipation symptom scores, BMI, fasting plasma glucose (FPG), glycosylated hemoglobin (HbA1c), and lipid profile were determined at the beginning and end of the 4-, 8-, and 12-week periods. After the 12-week intervention, constipation symptom scores, weight (−3.8 kg), BMI (−1.5 kg/m²), fasting plasma glucose (−26.7 mg/dL), cholesterol (−37.3 mg/dL), triglycerides (−10.4 mg/dL), LDLC (−21.0 mg/dL), and HDLC (+4.7 mg/dL) significantly decreased from baseline in the flaxseed group (all P-values < 0.05).

A 2025 parallel randomized clinical trial in Mexico: consuming 16 g of flaxseed daily for three months led to a significant decrease in glucose, total cholesterol, and triglyceride levels (p < 0.001) in Type 2 diabetes patients in the intervention group (n = 82).

A 12-week study in pre-diabetic participants: 25 overweight or obese patients with pre-diabetes were randomized into three groups. Participants consumed either 0 g, 13 g, or 26 g of flaxseed per day for 12 weeks. Fasting glucose, insulin, adiponectin, CRP, and HOMA-IR were determined at the beginning and end of the study. The consumption of flaxseed can significantly improve glycemic control, cytokines, and adipokines in overweight and obese participants with pre-diabetes.

Regarding SDG specifically: Pan et al. reported more modest but statistically significant improvements in glycemic control in type 2 diabetic patients treated for 12 weeks with 360 mg/day of flaxseed-derived lignan supplement.

Evidence strength: Multiple small-to-moderate-sized RCTs show favorable effects on fasting blood glucose. Evidence for HbA1c reduction is less consistent. Most trials are short (≤12 weeks) and conducted in specific populations (e.g., those with existing type 2 diabetes or pre-diabetes), limiting generalizability.

5.4 Cancer — Breast Cancer

In animal studies, alpha-linolenic acids have been shown to be able to suppress growth, size, and proliferation of cancer cells and to promote breast cancer cell death. Other animal studies found that the intake of flaxseed combined with tamoxifen can reduce tumor size to a greater extent than taking tamoxifen alone. Additionally, some clinical trials showed that flaxseed can have an important role in decreasing breast cancer risk, mainly in postmenopausal women.

In several studies of healthy women consuming flaxseed daily, estrogen levels decreased or estrogen shifted to more of a relatively inactive form. The result was less estrogen in the form that seems to promote breast cancer growth. However, studies show unexplained variability, which may partly reflect effects of individual differences in hormones, overall diet, and genetics.

Epidemiologic data of flaxseed dietary intake and high enterolactone serum concentrations — as well as in-human biomarker studies conducted in a randomized controlled design — suggest that flaxseed consumption may reduce the risk of breast cancer.

Evidence strength: Preclinical evidence is robust. Human evidence is based on biomarker studies and epidemiological associations rather than long-term incidence trials. Further studies are needed, specifically clinical trials that may demonstrate the potential benefits of flaxseed in breast cancer.

5.5 Cancer — Prostate Cancer

A key presurgical clinical study: the dose of 30 g (~3 rounded tablespoons) of ground flaxseed per day was chosen based on positive effects observed with an identical dose in pilot studies among men with prostate cancer, as well as a similar dose of 25 g used successfully in a clinical trial among women with breast cancer. The observation of lower proliferation rates with flaxseed supplementation is consistent with in vitro work in LNCaP, DU-145, and PC-3 prostate cancer cell lines, which also found inhibited cell growth with exposure to flaxseed-derived lignans.

A meta-analysis of case-control and cohort studies that included 11,346 cases and 140,177 controls found no association between enterolactone, matairesinol, secoisolariciresinol, or total lignans and prostate cancer. A systematic review came to a similar conclusion: most epidemiologic studies found no association between prostate cancer risk and dietary intake of flaxseed lignans or serum levels of enterolactone. The data are less clear for prostate cancer, but biomarker studies suggest that consumption of 30 g per day of flaxseed can reduce PSA levels and tumor proliferation rate.

Population studies are not clear about the impact of ALA on prostate cancer risk, but most recent overall analyses show no significant effect.

Evidence strength: Clinical studies show promising biomarker effects (reduced PSA, reduced tumor proliferation rate), but epidemiologic studies have not confirmed a reduction in prostate cancer incidence. Larger studies are needed to validate the effect of flaxseed consumption on cancer prevention or treatment.

5.6 Gastrointestinal Health and Constipation

Flaxseed's soluble mucilage and insoluble fiber content both contribute to bowel function. In a single-blinded, randomized controlled trial, 53 constipated patients with T2D received either 10 g of flaxseed pre-mixed in cookies twice per day or placebo cookies for 12 weeks. After the 12-week intervention, constipation symptom scores improved significantly in the flaxseed group compared to placebo (all P-values < 0.05).

NCCIH recently funded preliminary research to learn more about how flaxseed could affect the gut microbiome and inflammation in the body.

Evidence strength: Evidence for laxative and stool-normalizing effects is consistent across multiple trial types, though many studies combine this outcome with metabolic endpoints. The fiber mechanism is well-established physiologically.

5.7 Menopausal Symptoms

Preliminary data suggest that flaxseed, a rich source of dietary lignans, may be a potentially effective treatment for hot flashes. A phase III randomized, placebo-controlled trial was conducted to evaluate the efficacy of flaxseed in reducing hot flashes. Postmenopausal women with or without breast cancer were randomly assigned to a flaxseed bar (providing 410 mg of lignans) for 6 weeks versus a placebo bar. Participants completed daily, prospective hot flash diaries during the baseline week, and then ate one study bar per day for 6 weeks while recording their daily hot flashes.

However, a systematic review examined results from 11 controlled clinical trials with 632 participants with respect to flax and menopausal hot flashes. Included studies ranged in length from 6 weeks to 12 months. Flaxseed amounts utilized in the included trials ranged from 7.5 to 45 g daily. Although several of the included trials reported significant reductions in either frequency or severity of hot flashes, reductions were not significantly different from those seen with controls. Of five studies that compared flax to placebo, none demonstrated a benefit for flax that was different from the effect seen with placebo.

In contrast, a more recent trial found: following three months of supplementation, the intervention group showed substantially lower menopausal symptoms (p < 0.001), and enterodiol and enterolactone levels were considerably higher (p < 0.001). Therefore, flaxseed might ameliorate the symptoms associated with perimenopause. This study showed beneficial effects of flaxseed intervention on perimenopausal patients after three months of supplementation. However, this single-blind study has limitations, including that it involves findings observed for only three menstrual cycles.

Evidence strength: The aggregate evidence for flaxseed reducing hot flashes does not clearly separate from placebo in well-designed RCTs. Individual trials suggest possible benefit, especially for perimenopausal rather than postmenopausal women, and in those with higher lignan metabolizer status. Current evidence is mixed and inconclusive.

5.8 Inflammation

SDG metabolites may protect against CVD and the metabolic syndrome by reducing lipid and glucose concentrations, lowering blood pressure, and decreasing oxidative stress and inflammation. The majority of studies demonstrate that SDG interferes with the development of different types of diseases like cardiovascular, diabetic, lupus nephritis, bone, kidney, menopause, reproduction, mental stress, immunity, atherosclerosis, and urinary disorders due to its various biological properties, including anti-inflammatory, antioxidant, antimutagenic, antimicrobial, antiobesity, antihypolipidemic, and neuroprotective effects.

The anti-inflammatory effects of flaxseed have been reported inconsistently, and the lack of a strong anti-inflammatory response could limit its preventive impact on cancer development.

Evidence strength: Anti-inflammatory effects are well-documented in cell and animal models. Human clinical evidence is less consistent, with results varying substantially by population, dose, and outcome measure used.

6. Body Systems and Health Areas of Association

• Cardiovascular system: Lipid profile modulation (LDL-C, total cholesterol, triglycerides, Lp[a]), blood pressure reduction, antiatherogenic effects.
• Endocrine system: Phytoestrogenic modulation via enterolignans; effects on sex hormone metabolism; association with perimenopausal and postmenopausal symptom management.
• Gastrointestinal system: Soluble fiber and mucilage improve bowel regularity; effects on gut microbiota composition and metabolism of lignans.
• Metabolic/glycemic regulation: Reduction of fasting blood glucose, insulin sensitivity improvements, and possible HbA1c reduction in diabetic and pre-diabetic individuals.
• Oncology — hormone-sensitive cancers: Inhibition of cancer cell proliferation (breast, prostate), alteration of sex hormone metabolism, and reduction of tumor biomarkers in presurgical studies.
• Immune/oxidative stress: Antioxidant and radical-scavenging activity via SDG and its mammalian lignan metabolites; inhibition of platelet-activating factor.

Flaxseed is regarded as a functional food due to its high concentration of bioactive compounds such as polyunsaturated fatty acids (omega-3 and omega-6), vitamins, minerals, peptides, phenolic acids, flavonoids, and lignans. These bioactives have been associated with hypolipidemic, anti-atherosclerotic, anti-cholesterolemic, anti-inflammatory, and anticancer effects, among others.

7. Dosage Forms and Doses Used in Studies

The following doses reflect those specifically reported in published scientific research and should not be interpreted as established recommended intakes:

• Whole or ground flaxseed — cardiovascular risk factors: 40 g/day of ground flaxseed-containing baked products for 10 weeks in hypercholesterolemic adults.
• Whole flaxseed — postmenopausal women: 38 g/day of whole flaxseed for six weeks in postmenopausal women.
• Flaxseed powder — dyslipidemia and hypertension: 36 g of flaxseed sachet daily for 8 weeks.
• Flaxseed in baked goods — constipation and T2D: 10 g of flaxseed pre-mixed in cookies twice per day (20 g/day total) for 12 weeks.
• Flaxseed — prostate cancer (presurgical): 30 g (~3 rounded tablespoons) of ground flaxseed per day, with a stepped dose beginning at 10 g for days 1–3, 20 g for days 4–6, and 30 g from day 7 onward.
• Flaxseed — pre-diabetes: 25 overweight or obese patients consumed either 0 g, 13 g, or 26 g of flaxseed per day for 12 weeks.
• Flaxseed — type 2 diabetes (Mexico RCT): 16 g of flaxseed daily for three months.
• Flaxseed bar — menopausal hot flashes: one flaxseed bar providing 410 mg of lignans per day for 6 weeks.
• Flaxseed — menopausal symptoms (range across trials): flaxseed amounts utilized in included trials ranged from 7.5 to 45 g daily.
• SDG lignan extract — glycemic control: 360 mg/day of flaxseed-derived lignan supplement for 12 weeks in type 2 diabetic patients.
• SDG as a threshold for cardiovascular benefit: the current evidence suggests that a dose of at least 500 mg SDG/day for approximately 8 weeks is needed to observe positive effects on cardiovascular risk factors in human patients.
• SDG lignan extract (specific product): a 600 mg dose of a specific flaxseed lignan extract has been taken three times daily for 12 weeks.
• Ground flaxseed — perimenopausal symptoms: 10 g of flaxseed daily was used in one single-blind, randomized, placebo-controlled trial.

8. Safety Considerations and Interactions

General Safety

Limited amounts of flaxseed (especially flaxseed lignan extract or mucilage) or oral flaxseed oil supplements may be safe for use in the short-term (up to 6 months). Higher doses of flaxseed or flaxseed oil supplements may cause uncomfortable digestive symptoms like bloating, fullness, and diarrhea.

Cyanogenic Glycosides

Raw or unripe flaxseeds should not be eaten, as they may contain potentially toxic compounds. Flaxseed is known to contain cyanide-containing compounds known as cyanogenic glycosides, those found in flaxseed being liminarin, linustatin, and neolinustatin. These compounds, if consumed in excess over a long period of time, can result in goitrogenic problems and damage to other human organs. Despite numerous health benefits, flaxseeds also represent concerns due to excessive consumption and possible contamination, particularly from cyanogenic glycosides.

Thyroid Effects

Upon hydrolysis, cyanogenic glycosides release hydrogen cyanide, which in turn is converted to thiocyanates. Thiocyanates impede iodine uptake by the thyroid gland, and long-term exposure can exacerbate iodine-deficiency disorders such as goiter and cretinism.

Drug Interactions — Anticoagulants and Antiplatelet Drugs

There are theoretical reasons to suspect that flaxseed or flaxseed oil might interact with other drugs, such as anticoagulant or antiplatelet drugs. Flaxseed contains 4.3 µg of vitamin K per 100 g, so the vitamin K content is quite low relative to the recommended intake level. The omega-3 alpha-linolenic content of flaxseed oil has also been suggested to affect the international normalized ratio (INR; a measure of blood clotting) when taking warfarin.

Interaction with Metoprolol

There is some evidence that flaxseed might interfere with the body's ability to take in and use metoprolol. It is not known, though, if this interaction is clinically important.

Allergic Reactions

Case reports of anaphylaxis following ingestion of flaxseeds have been documented.

Pregnancy

Flaxseed and its lignan extracts appear to be safe for most adult populations, though animal studies suggest that pregnant women should limit their exposure.

Hormone-Sensitive Conditions

Because flaxseed's lignan metabolites act as weak phytoestrogens, individuals with hormone-sensitive conditions — including certain types of breast or uterine cancer, endometriosis, or uterine fibroids — should note the potential for hormonal effects. An increase in the luteal phase of the menstrual cycle after flaxseed supplementation has been reported in case reports.

Absorption of Oral Medications

The high soluble fiber and mucilage content of flaxseed has the theoretical potential to slow the absorption of orally administered drugs. This mechanism is not flaxseed-specific but applies to high-fiber foods and supplements generally. Spacing flaxseed consumption away from oral medications by at least one hour is sometimes recommended on a precautionary basis, though definitive clinical evidence for this effect with flaxseed specifically is limited.

References

• PMC9602266 — Flaxseed Bioactive Compounds: Chemical Composition, Functional Properties, Food Applications and Health Benefits-Related Gut Microbes
• PMC11945857 — Flaxseed in Diet: A Comprehensive Look at Pros and Cons
• ScienceDirect — Flaxseed (Linum usitatissimum L.) bioactive compounds and peptide nomenclature: A review
• ScienceDirect — Flaxseed (Linum usitatissimum); phytochemistry, pharmacological characteristics and functional food applications
• PMC4517353 — Potential protective properties of flax lignan secoisolariciresinol diglucoside
• PMC6299007 — Secoisolariciresinol Diglucoside of Flaxseed and Its Metabolites: Biosynthesis and Potential for Nutraceuticals
• PubMed 20003621 — Health effects with consumption of the flax lignan secoisolariciresinol diglucoside
• PMC6630319 — Flaxseed Lignans as Important Dietary Polyphenols for Cancer Prevention and Treatment: Chemistry, Pharmacokinetics, and Molecular Targets
• PMC2989356 — The cardiovascular effects of flaxseed and its omega-3 fatty acid, alpha-linolenic acid
• PMC3361740 — Meta-analysis of the effects of flaxseed interventions on blood lipids
• PubMed 18460483 — Flaxseed and cardiovascular risk factors: results from a double blind, randomized, controlled clinical trial
• PMC12068798 — Efficacy of flaxseed in reducing blood pressure among patients with cardiovascular risk factors: A systematic review and meta-analysis
• PMC6941378 — Effect of Flaxseed Powder on Cardiovascular Risk Factor in Dyslipidemic and Hypertensive Patients
• PMC5944250 — A randomized trial of the effects of flaxseed to manage constipation, weight, glycemia, and lipids in constipated patients with type 2 diabetes
• PMC9914786 — The Role of Flaxseed in Improving Human Health
• PMC11858396 — Flaxseed Improves Glucose and Lipid Metabolism in Mexican Subjects with Type 2 Diabetes: A Parallel Randomized Clinical Trial
• PMC2703189 — Flaxseed Supplementation (not Dietary Fat Restriction) Reduces Prostate Cancer Proliferation Rates in Men Presurgery
• PMC5808339 — The Effect of Flaxseed in Breast Cancer: A Literature Review
• PMC11818310 — Anti-Oxidant and Anti-Cancer Properties of Flaxseed
• Cancer Therapy Advisor — Flaxseed Lignans and Cancer
• American Institute for Cancer Research — Flaxseed and Cancer
• Memorial Sloan Kettering Cancer Center — Flaxseed
• PMC3246070 — A Phase III, Randomized, Placebo-Controlled, Double-Blind Trial of Flaxseed for the Treatment of Hot Flashes: NCCTG N08C7
• PMC11448742 — Effects of Flaxseed on Perimenopausal Symptoms: Findings From a Single-Blind, Randomized, Placebo-Controlled Study
• PMC11953605 — Dietary Status and the Effect of Flaxseed Supplementation on the Severity of Perimenopausal Symptoms
• SAGE Journals — Botanical Medicines and Phytonutrients in the Management of Menopausal Hot Flashes (2024)
• NCCIH — Flaxseed and Flaxseed Oil: Usefulness and Safety
• PMC6222892 — Bioprocessing of Functional Ingredients from Flaxseed
• PMC12608315 — Compositional and Functional Analysis of Golden and Brown Flaxseed: Nutrients, Bioactive Phytochemicals, Antioxidant Activity, and Cellular Responses
• ResearchGate — Bioavailability of Alpha-Linolenic Acid in Subjects after Ingestion of Three Different Forms of Flaxseed
• ResearchGate — Introduction: History of the cultivation and uses of flaxseed
• Austin Publishing Group — Chemical Composition and Health Benefits of Flaxseed