Horsetail (Equisetum arvense): Uses, Evidence and Safety

Key points first

• Other horsetail species differ. A European assessment report says Equisetum palustre is toxic, that adulteration of horsetail with it is possible, and that correct plant identification is important. [1]
• Horsetail contains an enzyme, thiaminase, that destroys thiamine. Canada requires manufacturers to prove their Equisetum arvense products are free of thiaminase activity, because it can lead to irreversible brain damage in people who are already thiamine deficient. [1]
• The EU herbal monograph lists conditions where reduced fluid intake is recommended, for example severe heart or kidney disease, as a contraindication for the urinary use. It also says use in pregnancy and breastfeeding is not recommended. [2]
• Evidence that horsetail helps hair or nails: Proper evidence not found (see the table).
• If someone has eaten a plant they cannot identify, call Poison Help at 1-800-222-1222 (US) or see plant and mushroom poisoning. For other countries see poison help by country.

Overview

Horsetail has been used in traditional European medicine for kidney and bladder complaints, wounds and many other purposes. The EMA report says it contains about 5 to 7.7% silicic acid or silicates. [1] This page is about the herb and what is claimed for it. It does not give identification, foraging or preparation instructions. For diuretic teas as a group, see diuretic teas and fluid balance.

Names, plant part and look-alikes

The accepted scientific name is Equisetum arvense L., in the horsetail family (Equisetaceae). [3] The EMA report lists synonyms Allosites arvense Brongn. and Equisetum boreale (L.) Borner, and says the drug is the whole or cut dried sterile aerial parts. [1] The genus Equisetum has 41 recognized species, and sixteen are documented in traditional medicine. [4] Adulteration with E. palustre, E. hyemale, E. fluviatile and E. sylvaticum is possible, and the European Pharmacopoeia includes a test for the absence of E. palustre. [1] The EMA report advises that only quality-controlled horsetail should be consumed because of this. [1] Because look-alikes exist, this page gives no identification advice. If you have a question about a wild plant, ask an expert local botanist or extension service.

Traditional use (not proof of effect)

The EMA report describes a broad traditional use, for example to promote elimination of water in kidney and bladder conditions, to stop bleeding, for fissured nails and loss of hair, and for poorly healing wounds. [1] The EU monograph lists two traditional herbal uses, based only on long-standing use: to increase the amount of urine to flush the urinary tract as an adjuvant in minor urinary complaints, and for supportive treatment of superficial wounds. [2] A 2023 review says species of Equisetum are widely used in traditional practices worldwide for genitourinary and rheumatic problems, hypertension and wound healing. [4] Traditional use is a record of use, not proof of effect.

What research shows

Forms and why the form matters

The EU monograph lists teas made from the cut herb, expressed juice, liquid extracts, dry extracts and preparations for use on the skin. [2] The EMA report says the high silica content of the plant is hard to extract by making tea in the usual way. [1] It also says no thiaminase activity was observed in the industrial aqueous dry extract and the fluid extract made with 30% ethanol that it examined, while the dried plant itself showed high thiaminase. [1] This is not a how-to. It shows that the form matters, and that a label saying "thiaminase-free" is something to check with a pharmacist.

Safety

• Thiamine: see above. People who are already thiamine deficient are most at risk of harm, according to the Canadian position described in the EMA report. [1]
• Nicotine-like effects: the report says small amounts of nicotine are possible in the plant (contradictory information), and that people sensitive to nicotine may react. [1] In animals, especially horses, a thiamine-destroying factor has been linked to muscle weakness, weight loss, abnormal pulse, cold extremities and fever, similar to nicotine poisoning. [1]
• Fluids, heart and kidney disease: the EU monograph names conditions where reduced fluid intake is recommended, such as severe cardiac or renal disease, as a contraindication. [2] See herbs and kidney disease.
• Potassium: in the 36-man trial, potassium excretion did not change significantly. [1] The EMA report also describes a woman who collapsed with a serum potassium of 2.8 mEq/l and a low thiamine level after using an herbal weight-loss medication from Mexico of unknown plants, and her authors suggested high-dose horsetail could be responsible. The product's contents were unknown. [1] So potassium loss is a reported concern, not a proven effect.
• Lithium and diuretic medicines: the EU monograph says no interactions were reported. [2] The EMA report describes a woman on lithium who developed toxicity (lithium level 4.5 mmol/l) after 2 to 3 weeks of an herbal mix with several diuretic herbs, including Equisetum hyemale, a different horsetail species. The assessor said no conclusion for Equisetum arvense was possible from this case. [1] The authors in general advised that people on lithium who use diuretic herbs may get dehydration and lithium toxicity. [1] Ask the prescriber or pharmacist; see narrow-range medicines.
• Pregnancy, breastfeeding, children: the EU monograph says safety has not been established, that use in pregnancy and breastfeeding is not recommended, and that use in children under 12 is not recommended. [2]
• Side effects and allergy: mild stomach problems and allergic reactions such as rash and facial swelling have been reported. [2] A patient allergic to cooked carrot reacted while using horsetail, and the authors thought a similar protein might be involved. [1]
• Surgery, liver: not described in the sources we read. Tell the surgical team about any herb (supplements before surgery).
• Quality: see quality testing and certifications.