Water Pepper (Persicaria hydropiper)
1. Identity and Botanical Classification
Accepted Names and Synonyms
Persicaria hydropiper (L.) Delarbre (synonym: Polygonum hydropiper L.) belongs to the family Polygonaceae and has gained considerable attention due to its diverse phytochemistry and medicinal uses. Other synonyms recorded in the literature include Persicaria hydropiper (L.) Spach, P. hydropiper (L.) Opiz, and P. hydropiper (L.) H. Gross.
The species is commonly known as marsh-pepper smartweed, marsh-pepper knotweed, smartweed, or water pepper; it is also called la liao in China, bishkatali or pakarmul in Bangladesh, and daun senahun in Malaysia. Other common names include: smartweed, water smartweed, biting knotweed, marsh pepper, biting pepper, sickleweed, bog ginger, Wasserpfeffer (German), renouée poivre d'eau (French), pimienta del agua (Spanish), and various Nordic names including vasspepper (Norwegian) and bitterpilört (Swedish).
Taxonomic Family Context
The family Polygonaceae takes its name from the genus Polygonum, first coined by the French botanist Antoine Laurent. The words "poly" and "gonum" mean "many" and "joints," respectively. Polygonaceae comprises about 1,300 species distributed in 59 genera.
Botanical Description
Water pepper is an annual herb with an erect stem growing to a height of 20 to 70 cm. The leaves are alternate and almost stalkless; the leaf blades are narrowly ovate with entire margins fringed by very short hairs, tapering with a blunt apex. The leaves are covered with transparent essence glands. At the base of each leaf are ochreas — membranous structures that surround the stem like a sheath.
Natural Habitat and Distribution
A widespread species, Persicaria hydropiper is found in Australia, New Zealand, temperate Asia, Europe, and North America. The plant grows in damp places and shallow water. Water-pepper smartweed is native to Europe and has spread to all other continents; all parts of this plant have an acrid pepper-like flavor, and the plant has a long history of use in traditional medicine in Europe. It was also adopted by Native Americans as a medicinal and occasional food plant.
Common Forms and Preparations
Its aromatic scent, along with its peppery, pungent, and slightly bitter taste, renders it a viable culinary ingredient. The leaves find application as vegetables in soups, stews, and salads, while young shoots are used for savory pies. In pharmaceutical contexts, water pepper is prepared as liquid extracts, decoctions, infusions, and tinctures. One standardized pharmaceutical form consists of a liquid extract of the herb (Herba Polygoni hydropiperis) at a 1:1 ratio, using 70% ethanol as the extractant. Cultivated varieties are also eaten in East Asia for their pungent flavor.
2. Traditional and Historical Use
Europe
All parts of this plant have an acrid pepper-like flavor, and the plant has a long history of use in traditional medicine in Europe. Water pepper became known early on for its poignant, peppery taste and has been used as a spice and medicine since ancient times. As an herbal medicine, it was considered an effective cure for a variety of ailments, including toothache, epilepsy, gangrene, rheumatism, and gout. In 19th-century eclectic medicine as documented in King's American Dispensatory, water pepper was categorized as stimulant, diuretic, emmenagogue, antiseptic, diaphoretic, and vesicant. The infusion, in cold water, was found serviceable in gravel, colds, and coughs, and in bowel complaints.
Russia and Eastern Europe
In Russian popular medicine, the leaves are used to stop bleeding and to treat haemorrhoids. Water pepper has been used for a long time as a haemostatic agent in uterine bleeding, and for heavy and painful menstruation, abortion, and postpartum bleeding. The leaves of this plant have also been used to treat cancer, colds, and coughs, and the infusion has been prescribed for treating rheumatism, chronic ulcers, haemorrhoid, tympanitis, and erysipelas.
China and East Asia
The stalks of Polygonum hydropiper L. have been traditionally used in clinical practice for thousands of years in China to treat various inflammatory diseases. Water pepper is a hot-tasting spice known in China, Japan, and Europe. In Japan, water pepper is known as tade and is used as a spice in certain dishes; its leaves and seeds add a peppery flavor to food. In Chinese traditional medicine, Polygonum hydropiper has been applied to treat gastrointestinal disorders for thousands of years and has demonstrated clinical efficacy with minimal adverse reactions.
South and Southeast Asia
A careful review of the ethnomedicinal literature has indicated various reported uses of P. hydropiper in epilepsy, edema, inflammation, headache, rheumatoid arthritis, chill, colic pain, fever, joint pain, and other infectious diseases. Women in Assam (India) have traditionally used the powdered dry root of P. hydropiper to prevent unwanted pregnancies.
North America (Eclectic and Native American Use)
In 19th-century eclectic American medicine, Dr. Eberle found water pepper efficient in the dose of a teaspoonful of the saturated tincture, repeated 4 or 5 times a day, or from 2 to 5 grains of the aqueous extract, in amenorrhoea; its use reportedly caused an increase of body heat with a sense of fullness in the pelvic region. The infusion was also used in painful and tardy menstruation; in flagging labor pains with fatigue it was described as prompt and serviceable.
3. Key Chemical Constituents and Active Compounds
Primary Compound Classes
Flavonoids are the major group of phytochemical components, followed by drimane-type sesquiterpenes and sesquiterpenoids, as well as phenylpropanoids. Water pepper's main active constituents are flavonoids (quercetin, kaempferol, hyperosid, isorhamnetin, rhamnazin), coumarins, and tannins (mainly hydrolysable tannins). In addition, the plant contains minerals, vitamins A, C and K, bitter substances, rutin, organic acids, and essential oils.
Sesquiterpenes
The pungency of Persicaria hydropiper is produced by polygodial, a 1,4-dialdehyde derived from drimane terpenoids. Polygodial, a sesquiterpene dialdehyde, is responsible for the herb's fiery taste and insecticidal properties. Various extracts and fractions of P. hydropiper were found to contain drimane-typed sesquiterpenes such as 3-β-angeloyloxy-7-epifutronolide, 7-ketoisodrimenin, changweikangic acid A, dendocarbin L, (+)-fuegin, futronolide, polygonumate, and (+)-winterin.
Flavonoids
Flavonoids identified in the plant include (+)-catechin, (−)-epicatechin, hyperin, isoquercitrin, isorhamnetin, kaempferol, quercetin, quercitrin, rhamnazin, and rutin. The methanol extract of the leaves has been found to contain additional flavonoids including apigenin-7-O-glucoside and various galloyl-glycoside derivatives.
Phenylpropanoids and Phenolic Acids
Phenylpropanoid esters identified include hydropiperosides A and B, and vanicosides A, B, and E, as well as phenolic acids such as caffeic acid, chlorogenic acid, and p-coumaric acid. The 2024 PMC study found that P. hydropiper extract exhibits a diverse phenolic profile, including quinic acid (3.68 ± 0.37 mg/g DW) and gallic acid (1.16 ± 0.10 mg/g DW).
Other Compounds
Compounds present in P. hydropiper have been reported to have multiple effects: antifungal agents (polygodial and warburganal), anti-inflammatory agents (polygonolide), and antioxidants (hydropiperoside, rhamnazin, persicarin).
4. Established Mechanisms of Action
Anti-Inflammatory Pathways
Studies in rat models of intestinal inflammation have documented a downregulation of iNOS and levels of Cox-2, TNF-α, and IL-1β, while the protein expression of NF-κB was significantly modulated after administration of P. hydropiper aqueous extract. The findings suggest that the anti-inflammatory effects are closely related to inhibition of NF-κB signal pathways.
In models of ethanol-induced gastric injury in rats, P. hydropiper extract pretreatment attenuated gastric mucosal injuries in a dose-dependent manner, increased the activity of superoxide dismutase, glutathione peroxidase, and glutathione, and decreased malondialdehyde levels. Pretreatment also reduced the generation of TNF-α and IL-1β in gastric tissue by downregulating NF-κB expression. Flavonoids are considered the main effective components against gastric mucosal injury.
Hemostatic and Vascular Mechanisms
The main active constituents — flavonoids, tannins, and essential oils — help to reduce capillary permeability and increase blood clotting, which makes the extract useful for various bleeding, including postpartum and menstrual bleeding.
TRPV1 Receptor Interaction (Polygodial)
Similar to capsaicin, systemic administration of polygodial and drimanial produces marked antinociceptive, anti-inflammatory, and antiallergic effects. These compounds also exhibit pungency on the human tongue and inhibit specific [3H]-RTX binding in rat spinal cord. This points to an interaction with the transient receptor potential vanilloid-1 (TRPV1) receptor system.
Antioxidant Mechanisms
Extracts of P. hydropiper can inhibit neutrophil infiltration, decrease pro-inflammatory factor levels (such as TNF-α and IL-1β), and increase the activity of antioxidant enzymes, as indicated by molecular pharmacology studies.
5. Scientific Evidence by Area of Use
5.1 Anti-Inflammatory Activity
Preclinical (animal/in vitro) evidence — preliminary
In a controlled rat study, aqueous extract of P. hydropiper stalks was administered at doses of 125, 250, and 500 mg/kg for 7 consecutive days to animals with TNBS-induced intestinal inflammation. Oral administration at the high dose significantly improved macroscopic score and histological parameters, ameliorated myeloperoxidase activity, and improved glutathione content. This is preclinical rodent data only; no human clinical trials of anti-inflammatory efficacy have been published.
5.2 Gastroprotective Activity
Preclinical evidence — preliminary
Studies showed that P. hydropiper water extract has a preventive effect on gastric mucosa injury, and further study showed that the effect of ethanol extract was superior to that of water extract. However, the mechanisms of the ethanol extract in the treatment of gastric illness remained under investigation. In Chinese traditional medicine, P. hydropiper and Coptis chinensis have been applied to treat gastrointestinal disorders for thousands of years and have demonstrated clinical efficacy with minimal adverse reactions. This clinical claim derives from traditional use literature, not controlled clinical trials. All mechanistic evidence remains preclinical.
5.3 Antimicrobial Activity
In vitro evidence only — preliminary
Confertifolin isolated from the leaf essential oil of P. hydropiper showed strong/good antibacterial activity against Enterococcus faecalis (MIC 31.25 μg/mL) compared to the positive standard streptomycin (MIC 25 μg/mL), but did not inhibit the growth of several other tested bacteria including S. aureus, E. coli, and P. aeruginosa. Separately, polygodial was found to have moderate bactericidal action against Bacillus subtilis (MBC 100 μg/mL), Staphylococcus aureus (MBC 100 μg/mL), Escherichia coli (MBC 100 μg/mL), and Salmonella choleraesuis (MBC 50 μg/mL). These results are promising but limited to in vitro evaluation, and thus more studies regarding solubilities, absorption, blood distribution, and pharmacodynamics are needed.
5.4 Antifungal Activity
In vitro evidence only — preliminary
MIC values obtained for isolated compounds confertifolin and drimenol from P. hydropiper essential oil, and polygodial from its chloroformic extract, ranged between 0.39 and 125 µg/mL, comparable with standard drugs. Confertifolin isolated from the leaf essential oil was also found to have potent antifungal activity against Epidermophyton floccosum, Curvularia lunata, and Scopulariopsis sp. All evidence remains in vitro.
5.5 Hemostatic Activity (Uterine and Other Bleeding)
Traditional use; limited formal clinical evidence
Water pepper has been used for a long time as a haemostatic agent in uterine bleeding, and for heavy and painful menstruation, abortion, and postpartum bleeding. The flavonoids, tannins, and essential oils in the extract help to reduce capillary permeability and increase blood clotting, which makes the extract used for various bleeding including postpartum and menstrual bleeding. This mechanistic rationale stems from phytochemical analysis and traditional pharmacological descriptions; no randomized controlled trials in humans have been published on hemostatic endpoints.
5.6 Cytotoxic and Potential Anticancer Activity
In vitro and in vivo preclinical evidence — very preliminary
Polygonum hydropiper is traditionally known for its cytotoxic effects and several bioactive cytotoxic compounds have been isolated from it. One study aimed to isolate potential anticancer compounds from its most potent fractions and evaluate their anticancer potentials. Active fractions including chloroform and ethyl acetate were subjected to column chromatography for compound isolation. Purified compounds were tested for cytotoxicity against breast cancer cells (MCF-7), cervical cancer cells (HeLa), and NIH/3T3 fibroblast cell cultures using MTT assay. These are cell-culture studies only; no human data exist.
5.7 Antioxidant Activity
In vitro evidence — preliminary
Different extracts and plant parts showed pharmacological activities including antioxidant, antibacterial, antifungal, antihelminth, antifeedant, cytotoxicity, anti-inflammatory, antinociceptive, oestrogenicity, antifertility, antiadipogenicity, and neuroprotection. Antioxidant data are generated largely from cell-free radical scavenging assays and cell culture models. No human clinical trials have evaluated antioxidant outcomes specifically.
5.8 Neuroprotective Activity
Preclinical (transgenic animal models) — very preliminary
Studies cited in the literature (Tong et al., 2020, Frontiers in Pharmacology) investigated neuroprotective effects of P. hydropiper essential oils using transgenic animal models. In different studies, this plant was found to possess activities including anti-Alzheimer, antidementia, neuropharmacological, sedative, and anxiolytic effects, with minimal toxicity. All neuroprotective data to date are from animal or in vitro models.
5.9 Estrogenic and Antifertility Activity
Animal evidence only — preliminary
In one study, estrogenic effects of the crude root extract of Polygonum hydropiper on uterine protein were tested in ovary-intact and ovariectomized female albino rats. The methanolic crude extract was given at a dose of 1,000 mg/kg body weight per day. Oral administration was carried out for a period of 12 days from the onset of proestrus. Results showed similar protein bands in ovariectomized females treated with estradiol and with the crude root extract, with the root extract stimulating expression of more uterine proteins. The crude root extract of Polygonum hydropiper was found to mimic the effect of estradiol-17β in the uterine protein profiles of adult female albino rats. This remains animal-level evidence only.
5.10 Antidiabetic Activity
In vitro enzyme inhibition and molecular docking — very preliminary
Extracts of P. hydropiper have been reported to exhibit neuroprotective, gastroprotective, cytotoxic, and antimicrobial properties. Based on the antidiabetic potentials of the Polygonaceae family, phytochemicals isolated from P. hydropiper were subjected to enzyme inhibition and molecular docking studies for potential applications in type 2 diabetes mellitus. No human trials exist in this area.
Overall Evidence Assessment
Extracts and compounds from P. hydropiper showed diverse biological activities including anti-inflammatory, antioxidant, cytotoxic, and antimicrobial activities. However, very few studies were performed using animal models, and many of these studies also lacked proper experimental settings such as use of positive and negative controls; in most studies, very high doses of extracts were administered. As of the current literature, no robust, peer-reviewed, randomized controlled trials in humans have been published for any therapeutic indication of water pepper.
6. Body Systems and Health Areas of Association
- Cardiovascular / Hemostatic System: In folk medicine, the plant is used as a means to stop bleeding, especially haemorrhoidal or uterine bleeding. This is related to the plant's ability to increase blood clotting.
- Gastrointestinal System: Modern research shows that P. hydropiper has a variety of pharmacological activities such as anti-inflammatory, antioxidant, and gastric protective effect. Traditional use spans diarrhea, colic, ulcers, and haemorrhoids.
- Female Reproductive System: Water pepper has been used for a long time as a haemostatic agent in uterine bleeding, and for heavy and painful menstruation, abortion, and postpartum bleeding.
- Musculoskeletal System: Domestically, its decoction is used as diuretic, anti-rheumatic, anti-inflammatory, haemostatic, and to relieve toothache.
- Immune / Inflammatory System: Suppression of NF-κB signaling, downregulation of COX-2, iNOS, TNF-α, and IL-1β has been demonstrated in preclinical models.
- Nervous System: Polygodial and related sesquiterpenes interact with TRPV1 receptors and have been investigated for antinociceptive and neuroprotective effects in animal models.
- Endocrine / Reproductive System (Estrogen axis): Animal evidence supports an estrogenic effect, particularly of root extracts.
- Oncology (Preclinical): According to the ethnopharmacology and contemporary medicine practice, P. hydropiper has a record of antipathogenic, anticancer, and immunomodulatory effects, promoting beneficial gut microbiota growth and brain functioning. This remains at the laboratory/preclinical stage.
7. Dosage Forms and Reported Dosages
Standardized dosage information is sparse in the scientific literature. The following figures are those specifically stated in cited sources and should be understood as reflecting investigational or historical data, not clinical recommendations:
- Rat anti-inflammatory model (TNBS-induced intestinal inflammation): Aqueous stalk extract administered orally at 125, 250, and 500 mg/kg for 7 consecutive days.
- Rat estrogenicity model: Methanolic crude root extract at 1,000 mg/kg body weight per day, administered orally for 12 days.
- 19th-century eclectic medicine (historical): The saturated tincture at a dose of one teaspoonful, repeated 4 or 5 times a day, or from 2 to 5 grains of the aqueous extract, was documented for amenorrhoea by Dr. Eberle.
- Pharmaceutical extract form: A standardized liquid pharmaceutical extract uses a 1:1 ratio with 70% ethanol as extractant.
- Optimal extraction for flavonoids: The maximum pharmacological effect of bioflavonoids is achieved by extraction with 70% ethyl alcohol, at which concentration flavonoids dissolve in both free (aglycone) and bound (glycosidic) form.
8. Safety Considerations and Interactions
Irritant Properties
Although water pepper is used as a spice in many Asian countries, due to its irritant properties, P. hydropiper is recommended to be used medicinally with caution. The sesquiterpene polygodial, which is responsible for the plant's pungent taste, is a known irritant to mucous membranes.
Mutagenicity — Conflicting Data
Regarding mutagenicity, negative results have been reported in the Ames test using TA 100, TA 98, and TA 2637 strains of Salmonella typhimurium, and in a mammalian cell V79/HGPRT assay. However, another series of studies performed under the supervision of the Ministry of Health, Labour and Welfare of Japan demonstrated water pepper extract to be positive for mutagenicity in both the Ames test using S. typhimurium TA 100 and TA 98, and in the chromosomal aberration test using Chinese hamster-derived CHL/IU cells, although negative for micronuclei in mouse bone marrow cells. The mutagenicity data are therefore conflicting, and this area warrants attention.
Subchronic Toxicity
Prior to a key subchronic toxicity study, there were no in vivo toxicity data available for water pepper extract or polygodial. The study was performed to evaluate the toxicity profile of water pepper extract by dietary administration to F344 rats according to the OECD Test Guideline 408 for a repeated-dose 90-day oral toxicity study in rodents.
Estrogenic Activity
The crude root extract of Polygonum hydropiper mimics the effect of estradiol-17β in the uterine protein profiles of adult female albino rats. This estrogenic activity in animal models raises theoretical concerns for individuals with hormone-sensitive conditions, though human data are not available.
Antifertility Activity
Women in Assam (India) have traditionally used the powdered dry root of P. hydropiper to prevent unwanted pregnancies. This reported antifertility use, consistent with the detected estrogenic activity in animal experiments, has not been evaluated in controlled human studies.
Mutagenicity and Toxicity — Overall Assessment
Mutagenicity and acute and subchronic toxicities of the plant have been reported in the literature. P. hydropiper has tremendous medicinal properties that could further be investigated for the development of evidence-based herbal products. The overall toxicological profile remains incompletely characterized, particularly in humans. Although various research reports have shown diverse biological activities for extracts and compounds obtained from P. hydropiper, very few studies were performed using animal models. Many of these studies also lacked proper experimental settings such as use of positive and negative controls and selection of dose, as in most of these studies very high doses of extracts were administered.
Insecticidal Properties
P. hydropiper leaf extracts have been shown to produce significant mortality against rice plant hopper (Nilaparvata lugens) nymphal and adult stages in a dose-dependent manner, with LC50 values of 620 ppm against adults and 230 and 130 ppm for larger and smaller nymphs, respectively. This is noted as it reflects the potency of the plant's bioactive compounds at relevant concentrations.
References
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- Kong et al. (2018). "Anti-Inflammatory Effects of an Extract of Polygonum hydropiper Stalks on TNBS-Induced Intestinal Inflammation in Rats by Inhibiting the NF-κB Pathway." PMC5964420.
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