Purple loosestrife

Purple loosestrife, Lythrum salicaria L., is a tall perennial of the loosestrife family (Lythraceae) that lines ditches, pond margins, reed meadows and riverbanks across Europe and temperate Asia. The part used is the dried flowering shoot tip, known in pharmacy as Lythri herba. Its quality is set by a European Pharmacopoeia monograph, a written quality standard (number 1537), which requires a minimum of 5.0 percent tannins expressed as pyrogallol. Those tannins, chiefly C-glucosidic ellagitannins such as vescalagin, castalagin and the salicarinins, give the herb its dry, puckering taste and explain its traditional use as an astringent, a substance that tightens tissue.

In France a fluid extract is registered as a traditional herbal medicine for mild diarrhoea, on the basis of long-standing use alone, and the European Pharmacopoeia sets the quality standard for the dried herb. There is no monograph from the European Medicines Agency, Commission E or ESCOP, and no health claim is authorised in the EU. Controlled human trials are missing entirely. Set against a deep European tradition, reaching from the classical materia medica through French and German medicine into the twentieth century, the modern research consists of laboratory and animal work alone. Purple loosestrife is a forgotten medicinal plant whose reputation rests on centuries of practice.

The record begins in antiquity, and it begins with a problem of names. Pliny reports that Erasistratus already used the plant against eczema, and Dioscorides recommends it for spitting of blood, for dysentery and to stop heavy menstrual bleeding. Yet Dioscorides describes his lysimachion with red or golden flowers. The name covered more than one plant, and later botany divided it between Lythrum salicaria of the Lythraceae and Lysimachia vulgaris of the Primulaceae, species from two unrelated genera. The French name for purple loosestrife, lysimachie rouge, still carries that old confusion. The classical records are real but leave the species open. In 1548 William Turner separates the yellow-flowered kind, which he attributes to Dioscorides, from a purple-flowered kind growing by water that Pliny describes. He names the purple one red loosstryfe or purple losestryfe. His editor, James Britten, identified it in 1881 as Lythrum salicaria.

What followed was decline in one country and survival in another. Albrecht von Haller's medical lexicon of 1755 lists it only as an external wound herb. Hecker's practical materia medica of 1815 still lists diarrhoea, dysentery and blood-spitting but notes that the remedy was by then rarely used. French medicine kept it alive. Fouquet devoted a memoir to the salicaire in 1793 and used it as a strong remedy for diarrhoea. Campardon confirmed his findings in 1883 and valued the plant for its tannin and mucilage. Henri Leclerc wrote that an infusion of the fresh plant served him well against stubborn diarrhoea in the field during the First World War. In German-speaking rural districts the plant lived on more in local names than in the medicine chest. These included Katzenwedel and Fuchswedel in Baden, Blutkraut on the Swabian Alb and Gullerwadel in Alsace. Switzerland had names of its own. Madaus read the name Stolzer Heinrich (Proud Henry) as a trace of the old belief that goblins, sometimes friendly and sometimes hostile, lived in the plant. He set the whole record down in 1938 and also named the physician De Haen, who recommended the plant against diarrhoea. In North America, where the plant had meanwhile naturalised, the Eclectics listed it in King's American Dispensatory in 1898. They were a school of American physicians who worked mainly with plant remedies. Then it faded. In 2015 Piwowarski and colleagues called it an underestimated medicinal plant whose popularity had declined for unknown reasons.

Tannins are central to the traditional use of purple loosestrife. The European Pharmacopoeia standardises Lythri herba on total tannins, at least 5.0 percent expressed as pyrogallol. Studies of its chemistry have shown that these are mainly C-glucosidic ellagitannins, a group of tannins. They include vescalagin and castalagin as single units, called monomers, and salicarinins A, B and C as paired units, called dimers. Further tannins include the ellagitannin pedunculagin and gallotannins such as the galloylglucoses. The herb also contains C-glucosidic flavones (vitexin, isovitexin, orientin and isoorientin), phenolic acids including gallic, ellagic and p-coumaric acid, anthocyanins, heteropolysaccharides, mucilage and sterols such as beta-sitosterol. Tannins are astringent, meaning that they tighten tissue, because they bind proteins and cause them to clump together. This tightens the surface of an irritated mucous membrane and reduces secretion. That is the classical rationale for an astringent tea and an external wash, and it has never been tested in a human trial.

The laboratory findings all come from cell culture, isolated tissue and animals, and they do not all point the same way. In the test tube, the aqueous extract and the isolated C-glucosidic ellagitannins damped several pro-inflammatory functions of human neutrophils, white blood cells of the first line of defence. The extract and the ellagitannins cut the production of the messenger interleukin 8 and of reactive oxygen species, chemically reactive oxygen compounds. They also reduced the release of the enzymes elastase and myeloperoxidase and the amount of the surface marker CD11b. The dimeric salicarinins were clearly more active than the monomers. The authors themselves point out that ellagitannins are poorly absorbed and are converted by gut bacteria into urolithins, so what the cells meet in the dish is not what circulates in a person. On isolated guinea-pig small intestine, an extract made with water and ethanol made the gut muscle contract rather than relax, mainly through muscarinic receptors, the docking sites for the nerve messenger acetylcholine. The French product Salicairine, by contrast, damped induced contractions of isolated rat duodenum, curbed diarrhoea induced with castor oil in mice and helped the rat colon absorb more fluid. The authors of the Salicairine study think this fluid absorption may explain at least part of the effect. Whether the tea eases diarrhoea by relaxing the gut therefore remains open. Extracts also show antioxidant and antimicrobial activity in the test tube, and in rodents the extracts made with water or methanol, given by mouth, reduced experimentally induced swelling and pain responses. The proposed mechanism has been partly described in laboratory and animal studies but has not been tested in people.

No controlled trial of purple loosestrife in people has been published, for diarrhoea or for anything else. The 2015 review by Piwowarski, Granica and Kiss in the Journal of Ethnopharmacology is the standard survey of the plant. It documents the traditional uses in detail and concludes that modern pharmacological research is still insufficient to support them. The French registration, too, rests on long-standing use alone. Any benefit claim you meet for this herb therefore rests on tradition and laboratory work alone.

There is little research, and all of it comes from cells, tissue and animals. Brun and colleagues in 1998 tested the French product Salicairine. In mice it curbed diarrhoea induced with castor oil, in rats it helped the colon absorb more fluid, and on isolated rat duodenum it damped induced contractions. Granica and colleagues found in 2020, in cell culture, that an extract of the herb slowed the growth of diarrhoea-causing E. coli bacteria and their attachment to intestinal cells. The extract and its ellagitannins also helped the gut cells close into a tight layer, which the authors read as support for the use against diarrhoea. Bencsik and colleagues in 2013, by contrast, found that an extract made with water and ethanol made isolated guinea-pig intestine contract rather than relax. Tunalier and colleagues in 2007 found antioxidant activity for purple loosestrife extracts in the test tube. In rodents, extracts made with water or methanol and given by mouth reduced experimentally induced swelling and pain responses. In the test tube, Piwowarski and Kiss in 2015 showed that the aqueous extract and isolated C-glucosidic ellagitannins damp several pro-inflammatory functions of human neutrophils. They also pointed out that these compounds are poorly absorbed and are converted to urolithins by gut bacteria. Pawlaczyk and colleagues in 2010 isolated an acidic glycoconjugate, a sugar-bearing compound, from the plant. It completely blocked clot formation in the test tube and in blood taken from rats, yet promoted clotting in living animals, a contradiction the authors flag in their title. Virchea and colleagues in 2025 tested an extract of wild Transylvanian plants in fruit flies. At low concentrations more flies survived. High concentrations were toxic and could kill all the flies, and on a high-sugar diet even lower concentrations were toxic.

Forms and preparation

The traditional preparation is a simple infusion of the dried flowering shoot tips. The Medicinal Plant Lexicon describes 2 to 5 g of cut herb in about 150 ml of boiling water, strained after ten minutes and drunk between meals. These figures describe the traditional tea, not the fluid extract registered in France. The tea tastes dry and puckering rather than aromatic, because the herb is rich in tannins and has almost no essential oil. Older German and French practice also used a decoction and a hot infusion made from a larger quantity, for example 15 to 20 g of flowering tops with stems to half a litre of water, drunk over the course of a day. Externally, the same infusion was used cooled as a wash, a compress or a gargle. Commercially the herb is sold cut as Lythri herba, as a tincture, as dry extract in capsules and, above all in France, as a hydroalcoholic fluid extract of the flowering aerial parts. One such preparation is registered there as a traditional herbal medicine for mild diarrhoea and contains 28 percent alcohol by volume. That makes it dangerous for people with alcohol dependence. Avoid alcoholic drinks while taking this product. If you have liver disease or epilepsy, talk to your doctor before taking it. Because tannins bind protein, leave milk out of the tea if you want its dry, astringent character, and do not take it together with iron supplements or other tablets. The pharmacopoeia defines the identity and tannin content of the dried herb, not the finished preparations.

Dosage

There is no official dosage for the tea, and the usual figures are traditional. The Medicinal Plant Lexicon describes steeping 2 to 5 g of the finely cut herb in about 150 ml of boiling water for ten minutes, then straining it. A cup is traditionally drunk several times a day between meals. The fluid extract registered in France is dosed for adults at 1.5 to 3 ml (30 to 60 drops) three times a day, for two days at most. Madaus records fluid extract doses of 0.5 g for children after Schulz and 3 to 5 g for adults after Leclerc. He also gives Leclerc's own recipe of 15 to 20 g of flowering tops with stems as a hot infusion in half a litre of water, drunk through the day. The children's dose is history, not a recommendation. Use under 18 is not recommended. Keep tea use short too, and see a doctor if diarrhoea has not improved after two days.

Special populations

There are no safety data for pregnancy, breastfeeding or children and adolescents under 18, so use is not recommended in any of these groups. The French registration of the fluid extract advises against it for the same reason. People with a sensitive or easily irritated stomach may find that a tannin-rich tea makes symptoms worse. If you take anticoagulant or antiplatelet medicines, talk to your doctor first, because in a single animal study an isolated constituent promoted clotting. Anyone with persistent diarrhoea, blood in the stool, fever, or diarrhoea in a small child or an older person at risk of dehydration should see a doctor rather than reach for a herbal tea.

Drug interactions

No controlled interaction study exists. The French registration lists only interactions that follow from the alcohol in its fluid extract (28 percent by volume), for example with disulfiram, metronidazole and medicines that depress the central nervous system. Two further cautions follow from what is known about the plant. Tannins bind proteins, alkaloids and metal ions in the gut. A tannin-rich tea taken with a medicine or an iron or zinc supplement can therefore reduce how much of it is absorbed. Leave one to two hours between them. And in a single animal study, an isolated acidic glycoconjugate, a sugar-bearing compound, from the plant promoted clotting, while in the test tube it blocked clotting. This is a contradictory finding from laboratory and animal work, not an interaction observed in people. If you take warfarin, a direct oral anticoagulant that prevents clotting, or an antiplatelet medicine that stops platelets sticking together, tell your doctor that you use purple loosestrife. Concentrated extracts need the most care.

Contraindications

There is no European monograph listing contraindications. The points below draw on the French registration of the fluid extract and on the general pharmacology of tannins. Do not use purple loosestrife if you are hypersensitive to it. The French product is also contraindicated if you are allergic to any of its other ingredients. Pregnancy and breastfeeding: not recommended, because safety has not been established. Children and adolescents under 18: not recommended, because safety has not been established. Acute or chronic inflammatory bowel disease and known gastric irritation: a tannin-rich preparation can make symptoms worse and should be avoided without medical advice. Seek medical help immediately for severe or profuse diarrhoea, blood or mucus in the stool, fever, vomiting or other digestive symptoms. Have diarrhoea checked if it lasts more than two days. Any diarrhoea in a small child or a frail older person needs medical care. Do not treat this diarrhoea yourself.

Side effects

The French registration of the fluid extract lists no side effects, and the specialist literature reports none of note. Like other tannin-rich herbs, purple loosestrife can still irritate a sensitive stomach and cause nausea or stomach discomfort, especially on an empty stomach or in larger amounts. Allergic reactions are possible. Concentrated extracts need more care than tea, because in a 2025 study in fruit flies high extract concentrations were clearly toxic. In the Ames test, the standard bacterial test for genetic damage, the extract of the French product showed no effect. In a micronucleus test on freshwater mussels in 2015, however, an extract made by shaking dried leaves and stems with water for 24 hours caused statistically significant damage to genetic material, and the authors call for its safety as a medicinal herb to be assessed. That extract was neither the tea made from flowering shoot tips nor the French product. No studies of cancer risk or effects on reproduction exist for the product. Stop using it and get advice if anything unexpected happens.