Salicin
1. Identity and Chemical Characterization
Chemical Names and Classification
β-d-Salicin, or 2-(hydroxymethyl)phenyl-O-β-d-glucopyranoside, is the first phenolic glycoside discovered in nature, with a molecular mass of 286.27782 g/mol. Its IUPAC name is (2R,3S,4S,5R,6S)-2-(hydroxymethyl)-6-[2-(hydroxymethyl)phenoxy]oxane-3,4,5-triol. It is classified as an aryl β-D-glucoside that is salicyl alcohol in which the phenolic hydrogen has been replaced by a β-D-glucosyl residue. Other names in common or chemical use include D-Salicin, β-D-Glucopyranoside 2-(hydroxymethyl)phenyl, Salicine, Salicoside, Salicyl alcohol glucoside, and Saligenin-β-D-glucopyranoside (CAS No. 138-52-3).
The chemical structure of β-d-salicin encompasses β-d-glucose and 2-hydroxybenzyl alcohol (salicyl alcohol). β-d-Salicin contains seven oxygen atoms as hydrogen-bond acceptors and five hydroxyl groups as hydrogen-bond donors. These chemical features contribute to the polarity of the molecule, which is why extraction requires a polar solvent system, such as boiling water or ethyl alcohol. Salicin has an aryl beta-D-glucoside chemical structure where the phenolic hydrogen has a beta-D-glucosyl residue substituted and is classed as a benzyl alcohol.
D(-)-Salicin is an organic compound with the chemical formula C₁₃H₁₈O₇, extracted from white willow bark; it is a glycoside compound. When first isolated in 1828 by German chemist Joseph Buchner, the substance was described as yellowish and bitter-tasting, and he named it salicin after salix, the Latin word for willow.
Natural Sources
Comminuted or powdered barks from Salix (willow) species, especially S. alba, S. nigra, S. purpurea, S. daphnoides, and S. fragilis, are well-known phytomedicines with a history of ethnomedical use. D(-)-Salicin has been found to be widely present in the bark and leaves of a wide range of willow and poplar plants; for example, purple willow bark (S. purpurea) contains up to 30% salicin. Salicin is the precursor of salicylic acid, comprising approximately 1% of the white willow bark extract, whereas other glycosides comprise about 12%.
Like other Salicaceae, the genus Populus (poplars) is characterized by a diversity of phenolics; in particular, salicin-based phenolic glycosides are found in the leaves and bark of all species examined. These phytochemicals consist of glucosides of salicyl alcohol, which are generally further esterified and benzoylated. Another natural salicylate, methyl salicylate, is found in wintergreen, birch tree, mango, meadowsweet, guelder-rose, and is used as an analgesic medicine and fragrance.
Common Forms and Preparations
White willow bark extracts are commercially available in various grades and compositions — for example, 15, 25, or 50 percent salicin. Willow is available in various dosage forms, including tablets, capsules, powders, and liquids. A proprietary extract of willow bark, Assalix, has been standardized to contain 15% salicin. In view of the considerable variation in salicylate concentrations between different Salix species, the salicin content of products should be quantified and declared.
2. Historical and Traditional Use
Ancient Civilizations
The use of willow extracts as a medicine was recorded in the Ur III stone tablet from ancient Sumeria (now part of modern Iraq) and the Ebers papyrus from ancient Egypt. These documents can be dated to the third millennium BCE. The Chinese used poplar tree (Populus alba L.) barks and willow (Salix babylonica L.) shoots for centuries to treat rheumatic fever, colds, haemorrhages, and goitre, and as a general antiseptic for wounds and abscesses. In the fourth century BCE, the father of medicine Hippocrates reportedly prescribed extracts of willow leaves or barks to reduce fever and pain during childbirth.
Historically, white willow bark has been used for more than 2,000 years, initially in Egypt and Greece, then in China, Europe, North and South America, the Caribbean, and the Mediterranean. Willow (and other salicylate-containing herbs) has been used as an analgesic for at least 2,000 years, recommended by physicians such as Paracelsus and Hippocrates, all the way up to the Eclectic physicians. Native Americans used numerous willow species for their analgesic, anthelmintic, and hemostatic properties.
Preparations in Traditional Use
Traditional preparations most commonly involved decoctions or infusions of the dried inner bark. Despite thousands of years of use, the medicinal properties of willow were not studied clinically until the 1700s, when Reverend Edward Stone (1702–1768), a fellow at Wadham College, University of Oxford, examined the use and efficacy of willow by drying the boiled willow water to produce a powdered version of the medicine. A significant step occurred in 1763 when Reverend Edward Stone reported to the Royal Society on his successful use of dried willow bark to treat fevers.
Purposes of Traditional Use
Due to its ability to alleviate pain and reduce inflammation, white willow has been traditionally employed to treat conditions such as headaches, muscle pain, and arthritis. Willow bark has been used for centuries as a treatment for pain, headache, and inflammatory conditions such as bursitis and tendinitis.
Isolation and Early Scientific History
In 1828, German and French pharmacists and chemists, including Johann Buchner and Henri Leroux, extracted and purified a bitter, yellow crystalline substance from willow bark, which they named "salicin" after Salix, the Latin name for willow. In 1829, Henri Leroux from France further developed salicin extraction, and nine years later Raffaele Piria refined the compound and produced salicylic acid. Salicin was used by Dr. Thomas MacLagan in a trial for treatment of rheumatic fever in Dundee, which was reported in The Lancet in 1876. In 1897, records from Felix Hoffmann working for Bayer document the formation of acetylsalicylic acid, which was soon after known as aspirin. The commercial name aspirin was derived from the chemical combination of "a" from the acetyl group and "spir" from the salicin group, which at that time was sourced from meadow (Spiraea sp.).
3. Key Constituents, Phytochemistry, and the Broader Salicinoid Family
Salicin as the Principal Marker Compound
Salicin, a prominent phenolic glycoside derived from Salix species, has traditionally been used to manage pain and inflammatory conditions. It is mainly salicin and the salicyl glycosides which form salicin after hydrolysis that represent salicylic acid pro-drugs; salicin and salicyl glycosides have antipyretic, analgesic, and anti-rheumatic properties.
Other Phytochemicals in Willow Bark
There are at least 13 different main compounds, including saligenin, salicylic acid, salicin, isosalicin, picein, salidroside, triandrin, salicoylsalicin, salicortin, isosalipurposide, salipurposide, naringenin-7-O-glucoside, and tremulacin, in willow bark identified and analyzed with the high-performance liquid chromatography technique and mass spectrometry. Although extracts of white willow bark are standardized usually to D(-)-Salicin, the extracts include other components such as salicylates, polyphenols, and flavonoids, which can also play a key role in therapy.
Plants in the Salicaceae family contain various secondary metabolites such as anthocyanins and phenolic glycosides (salicinoids) derived from salicin, a prodrug of salicylic acid. A metabolomics screen of 86 Salix species contained in the UK National Willow Collection led to the discovery, isolation, and full characterization of a new natural salicinoid — salicin-7-sulfate. This molecule may have important human pharmacological actions that need to be considered in determining the efficacy and safety of willow herbal medicines.
4. Mechanisms of Action
Prodrug Metabolism to Salicylic Acid
The therapeutic effects of willow bark primarily stem from salicin's role as a precursor to salicylic acid. Once ingested, salicin is metabolized in the body to salicylic acid, which exhibits anti-inflammatory, analgesic, and antipyretic properties. Pharmacokinetic studies indicate that formulation and gut microbiota significantly influence bioavailability, with rapid hydrolysis to saligenin followed by metabolism to salicylic acid. In its natural form, D(-)-Salicin can pass through the gastrointestinal system and is converted to salicylic acid in the blood and liver. The conversion process takes several hours, so the body does not feel the effects immediately, but the general effect lasts for several hours.
A key pharmacokinetic study published in European Journal of Clinical Pharmacology characterized the full metabolite profile in humans. The study evaluated the pharmacokinetics of salicin and its major metabolites in humans after oral administration of a chemically standardized willow bark extract. Willow bark extract corresponding to 240 mg salicin (1,360 mg) was ingested by ten healthy volunteers in two equal doses at times 0 h and 3 h, with urine and serum levels of salicylic acid and its metabolites — gentisic acid and salicyluric acid — determined using reverse-phase high-performance liquid chromatography. Salicylic acid was the major metabolite of salicin detected in the serum (86% of total salicylates), besides salicyluric acid (10%) and gentisic acid (4%). Peak levels were reached within less than 2 hours after oral administration.
COX Inhibition and Prostaglandin Suppression
Willow bark is used to relieve fever, sore throat, headache, and flu, mainly due to the presence of salicin, which is a natural nonselective COX-1 and COX-2 inhibitor. However, the precise contribution of salicin itself — versus whole-extract constituents — to COX inhibition is a subject of ongoing debate. Salicylic acid (the downstream metabolite) does not possess acetylating activity and does not inhibit cyclooxygenase in vitro; however, salicylic acid blocks cyclooxygenase expression at the transcriptional level, thereby explaining its anti-inflammatory properties. In addition, both salicylic acid and aspirin inhibit nuclear factor kappa B (NF-κB) activity by inhibiting IκB kinase β (IKKβ).
Analysis of blood samples from a small study of 3 patients receiving a single dose of willow bark extract equivalent to salicin 240 mg found only moderate inhibition of COX-1. Both salicin and salicylate alone had no effect in several inflammatory parameters in one study, suggesting that the proprietary Salix extract inhibits COX-2-mediated PGE₂ release through compounds other than salicin or salicylate.
Multi-Target Mechanisms: NF-κB, TNF-α, and COX-2
The clinical efficacy of willow bark extract cannot be explained by its salicin content alone, and different modes of action have been suggested for the anti-inflammatory effect. Aqueous willow bark extracts showed concentration-dependent and significant anti-inflammatory effects in lipopolysaccharide-activated monocytes. Both the extract and a water-soluble fraction inhibited the intracellular protein expression of tumour necrosis factor-alpha (TNFα) and the mRNA expression of TNFα and cyclooxygenase-2 (COX-2). Treatment of activated macrophages inhibited the nuclear translocation of the p65 subunit of NF-κB. In summary, these in vitro investigations suggest significant anti-inflammatory activity via inhibition of TNFα, COX-2, and NF-κB.
Willow bark extract significantly raises GSH (reduced glutathione) levels, an effect which helps limit lipid peroxidation, and was more potent than either aspirin or celecoxib in this regard. Antiproliferative effects in human colon and lung cancer cells are due to growth inhibition and apoptotic induction. In vitro and animal models suggest that salicin can inhibit ROS and ERK signaling pathways to produce antiangiogenic effects. These mechanistic findings are preliminary and derive primarily from cell or animal studies.
The Broader-Than-Salicin Question
As the precursor of aspirin, salicin cannot fully explain the clinical effect of willow bark; both flavonoids and polyphenols in the willow bark have been proven to be attributed to anti-inflammatory effects. The possible mechanism of anti-inflammatory action of proanthocyanidin dimers B1 and B2, also present in willow, may be connected with the inhibition of transcription of nuclear factor-kappa B (NF-κB).
5. Scientific Evidence by Area of Use
5.1 Low Back Pain
Evidence strength: Moderate (human RCT data; limited number of trials; small populations)
The most clinically investigated application of salicin-standardized willow bark extract is nonspecific low back pain. One landmark study was designed to evaluate the effectiveness of willow (Salix) bark extract, widely used in Europe, for the treatment of low back pain. The researchers enrolled 210 patients with an exacerbation of chronic low back pain who reported current pain of 5 or more (out of 10) on a visual analog scale. They were randomly assigned to receive an oral willow bark extract with either 120 mg (low dose) or 240 mg (high dose) of salicin, or placebo, with tramadol as the sole rescue medication, in a 4-week blinded trial. After four weeks, 39% of the high salicin group (n=65) were pain-free, 21% of the low salicin group (n=67) were pain-free, and only 6% of the placebo group (n=59) were pain-free. Moderate efficacy was demonstrated with both doses of salicin for short-term treatment of acute episodes of chronic nonspecific lower back pain.
In a comparative open trial, willow bark standardized extract delivering 240 mg salicoside daily was shown to be as effective as rofecoxib (Vioxx) 12.5 mg daily for relieving chronic low back pain in 114 patients.
The Cochrane Collaboration has evaluated this evidence. The Cochrane review found that willow bark, in a standardized daily dose of 120 mg and 240 mg of salicin, reduced pain more than placebo; a standardized daily dose of 240 mg reduced pain about the same as a daily dose of 12.5 mg of Vioxx® (a non-steroidal anti-inflammatory drug). The review found two moderate-quality trials utilizing Salix alba (white willow bark) that provided moderate evidence for short-term improvements in pain and rescue medication for daily doses standardized to 120 mg or 240 mg salicin, with an additional trial demonstrating relative equivalence to 12.5 mg per day of rofecoxib.
The 2009 systematic review in Phytotherapy Research similarly concluded that one confirmatory and two exploratory studies indicate a dose-dependent analgesic effect not inferior to rofecoxib in patients with low back pain. All studies investigated ethanolic extracts with daily doses up to 240 mg salicin over periods of up to six weeks; minor adverse events occurred during treatment. The review provides moderate evidence of effectiveness for the use of ethanolic willow bark extract in low back pain.
Despite these positive findings, the overall evidence base remains limited. Despite its long history of use, only a few small clinical trials have been conducted that support the use of willow bark extracts in chronic low-back pain and osteoarthritis.
5.2 Osteoarthritis
Evidence strength: Mixed/Limited (small RCTs; results inconsistent across trials)
A 6-week randomized, controlled, double-blind trial studied 127 outpatients with hip or knee osteoarthritis (OA) and a WOMAC pain score of at least 30 mm. Patients were randomized to receive willow bark extract corresponding to 240 mg of salicin/day, diclofenac 100 mg/day, or placebo (n=43, 43, and 41, respectively), with the main outcome being the pain subscore of the WOMAC OA Index. WOMAC pain scores decreased by 8 mm (17%) in the willow bark group and by 23 mm (47%) in the diclofenac group, compared with 5 mm (10%) in the placebo group. While the willow bark group showed modest improvement over placebo, the response was substantially lower than diclofenac, limiting its clinical significance.
In one exploratory and one confirmatory study, conflicting results were achieved in participants with osteoarthritis. A 2023 meta-analysis in PMC reached a more positive conclusion: a recent meta-analysis compiled and reviewed evidence from randomized controlled trials evaluating the efficacy and safety of willow bark preparations in both OA and RA. The meta-analysis results showed that compared to placebo, willow bark significantly reduced pain and improved health status in arthritis patients across the studies analyzed. However, the overall effect size was modest. The meta-analysis showed a statistically significant difference in pain controls for patients with arthritis between the willow bark and placebo groups (SMD: −0.31; 95% CI: −0.53, −0.08).
The evidence for osteoarthritis and related joint-discomfort settings is more limited and mixed. One study noted that although salicin derivatives in the willow bark were metabolized in vivo to salicylic acid, serum salicylate concentration was too low to reach clinical effects. Further studies are required to determine if treatment of osteoarthritis and rheumatoid arthritis requires extracts with higher doses than 240 mg salicin per day.
5.3 Rheumatoid Arthritis
Evidence strength: Weak/Insufficient (single underpowered trial; no significant effect)
In a 6-week randomized double-blind trial, 26 outpatients with active rheumatoid arthritis were randomized to receive willow bark extract corresponding to 240 mg salicin/day (n=13) or placebo (n=13), with the main outcome being patient-assessed pain on a 100 mm visual analog scale. No significant effect was seen in this confirmatory study in patients with rheumatoid arthritis, but the study was grossly underpowered. No adequately powered RCTs in rheumatoid arthritis have been published to date.
5.4 Fever Reduction (Antipyretic Effect)
Evidence strength: Primarily preclinical and historical; limited human RCT data for salicin specifically
D(-)-Salicin is used as a "natural aspirin" to treat minor fevers, colds, infections (flu), acute and chronic rheumatic discomfort, headaches, and pain caused by inflammation. The antipyretic rationale derives from the downstream conversion to salicylic acid, which shares fever-reducing properties with aspirin through prostaglandin suppression. It is mainly salicin and the salicyl glycosides which form salicin after hydrolysis that represent the salicylic-acid pro-drug; salicin and salicyl glycosides have antipyretic, analgesic, and anti-rheumatic properties according to EMA assessment. However, specific well-powered clinical trials isolating salicin's antipyretic effects in humans are lacking in the published literature.
5.5 Anti-Inflammatory Effects: Preclinical Evidence
Evidence strength: Preliminary; largely in vitro and animal data
In recent years, various in vitro and animal studies have demonstrated that the anti-inflammatory activity of white willow bark extracts is associated with down-regulation of tumour necrosis factor-α (TNF-α) and nuclear factor κB (NF-κB). Using SARS-CoV-2 peptide/IL-1β- or LPS-activated human PBMCs and an inflammatory intestinal Caco-2/HT29-MTX co-culture, Salix extracts and ASA concentration-dependently suppressed prostaglandin E₂ (PGE₂), a principal mediator of inflammation. These findings remain at the in vitro level and have not yet been validated in clinical trials.
Willow bark extract has the therapeutic effect of preventing oxidative stress and induces apoptosis in human colon and lung cancer cells. Other possible mechanisms of action proposed for salicylate activity include JNK pathway inhibition and direct allosteric activation of AMP kinase (AMPK). These findings are all preclinical; no clinical trials have evaluated salicin for cancer or metabolic indications.
6. Body Systems and Health Areas Associated with Salicin
- Musculoskeletal system: Willow bark extract is commonly employed as a complementary therapy for pain and inflammation management, such as those related to low back pain, osteoarthritis, tendinitis, bursitis, and headaches.
- Immune and inflammatory pathways: Crude willow bark extracts exert multiple biological actions, including suppression of pro-inflammatory cytokines, modulation of NF-κB signaling, and inhibition of cyclooxygenase (COX) enzymes.
- Thermoregulatory system: Salicin and its metabolite salicylic acid act on the hypothalamic-thermoregulatory pathways, consistent with the traditional use for fever reduction documented across multiple civilizations.
- Antioxidant and vascular: Willow bark extract was found to significantly raise low glutathione levels, thereby limiting lipid peroxidation. Given its antioxidant properties, salicin has been studied for its potential to prevent oxidative stress-related diseases, such as cardiovascular diseases and neurodegenerative disorders. These associations are based on preclinical data only.
- Gastrointestinal tract: Salicin must pass through the gut for hydrolysis and absorption, and gastrointestinal tolerability is a relevant safety consideration (see Section 8). Although willow bark extracts are generally standardized to salicin, other compounds in the extracts, including polyphenols, may also play prominent roles in areas such as cosmetology.
7. Dosage Forms and Dosages Reported in Clinical Studies
In a 4-week, double-blind clinical trial of patients with exacerbation of chronic low back pain (N=191), two oral doses of willow bark extract containing salicin 120 mg or 240 mg were compared with placebo; the primary outcome measure was the proportion of patients requiring rescue medication (tramadol) five out of seven days during the final week of the study.
In one randomized OA trial, the willow bark extract was standardized to 17.6% salicin, and each coated tablet contained 340 mg extract of willow bark, delivering 120 mg salicin per tablet (240 mg salicin daily). Another study used a willow bark extract standardized to 15.2% salicin, providing 60 mg of salicin per tablet, given at doses of 1–2 tablets per day or 3–4 tablets per day for 6–8 weeks.
Clinical studies evaluating the analgesic effects of willow bark (e.g., for lower back pain, dysmenorrhea) used extracts delivering daily salicin doses of 120 to 240 mg. All studies reviewed in one systematic review investigated ethanolic extracts with daily doses up to 240 mg salicin over periods of up to six weeks.
Metabolism of 240 mg salicin from willow bark could yield 113 mg of salicylic acid. Intake of willow bark extracts standardized to 240 mg salicoside per dose was found to lead to serum levels of salicylic acid consistent with intake of 87 mg of aspirin. This is notably lower than a standard aspirin dose of 500 mg, and points to the fact that salicin's clinical activity is not fully explained by the salicylic acid produced.
8. Safety Considerations and Drug Interactions
General Safety Profile in Clinical Studies
In the clinical studies reviewed, no serious adverse events were reported. In one observational study, patients (4.3%) reported 46 adverse drug reactions that were related to the GI tract (3.1%) and skin (1.6%), but none were classified as serious. Trials using willow bark extracts containing 120–240 mg salicin, the alleged active ingredient, reported no major adverse outcomes.
Contraindications
Willow-containing products should be avoided in patients with known hypersensitivity to aspirin, asthma, impaired thrombocyte function, need for vitamin K antagonistic treatment, diabetes, gout, kidney or liver conditions, peptic ulcer disease, and any other medical condition for which aspirin is contraindicated. The EMA monograph contraindicates the use of willow bark in patients with hypersensitivity to other nonsteroidal anti-inflammatory drugs and in people with asthma because of potential severe reactions (acute bronchospasms). This contraindication is maintained in the 2017 review carried out by the EMA.
Pregnancy, Lactation, and Pediatric Use
As there is no information on reproductive and developmental toxicity, the use during pregnancy is not recommended; tests on reproductive toxicity, genotoxicity, and carcinogenicity have not been performed. The United States Pharmacopeia has included a cautionary labeling statement in the USP Salix Species monograph: "Dosage forms prepared with this article should bear the following statement: 'Not for use in children, women who are pregnant or nursing, or by persons with known sensitivity to aspirin.'" The American Herbal Products Association's Botanical Safety Handbook classifies Salix spp. bark as an herb that can be safely consumed when used appropriately; however, caution is advised because of the risk of increased bleeding and the fact that salicylates cross the placenta and that newborns eliminate them very slowly.
Gastrointestinal Tolerability vs. Aspirin
Salicylic acid was found to be irritating to the mouth and stomach, and alternatives were sought. To ingest 1 g of salicin from willow bark, a person must ingest at least 14 g of the bark. Compared to NSAIDs, which include aspirin, white willow bark extracts have relatively few adverse effects. Aspirin (acetylsalicylic acid), as a synthetic alternative to D(-)-Salicin, has potentially dangerous gastrointestinal side effects.
Drug Interactions
In general, drug interactions associated with salicylates may apply to willow-containing products; however, actual salicylate content of willow species is likely low. Use should be avoided with alcohol, barbiturates, sedatives, and other salicylate-containing products because of potential additive irritant effects, including GI tract and platelet function adverse reactions.
Just like aspirin, white willow bark might increase the risk of bleeding, and so concomitant use of anticoagulants and antiplatelet drugs should be avoided. Drugs of concern include aspirin, clopidogrel (Plavix), dalteparin (Fragmin), enoxaparin (Lovenox), heparin, ticlopidine (Ticlid), warfarin (Coumadin), and others. A warning in case of concomitant use with anticoagulants is recommended in the EMA monograph.
Willow bark contains salicin, a plant salicylate. Human case reports suggest that a combination of acetazolamide and salicylate increases unbound plasma levels of acetazolamide as well as adverse effects related to acetazolamide; theoretically, willow bark might result in additive adverse effects associated with acetazolamide.
Regulatory and Monograph Status
According to the World Health Organization, Salix alba bark used in herbal medicines must conform to specific quality specifications; the product must comprise ≥1.5% of total salicylate derivatives expressed as salicin by high-performance liquid chromatography. Metabolism of 240 mg salicin from willow bark could yield 113 mg of salicylic acid, yet dietary supplement products are not required to be labeled with warnings. In contrast, over-the-counter low-dose aspirin (81 mg strength), which delivers 62 mg salicylic acid, is required by law to include cautions, warnings, and contraindications related to its use in pregnant and nursing women, children, and other vulnerable subpopulations. This regulatory gap has been noted by the United States Pharmacopeia Safety Review.
References
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