Horsetail extract is a herbal supplement made from the stems of Equisetum arvense, a plant that has been used in traditional medicine for centuries. It’s one of the richest plant sources of silicon, a mineral involved in building connective tissue, and it contains a range of antioxidant compounds. You’ll find it sold as capsules, liquid tinctures, teas, and as an ingredient in topical skin and nail products.
What’s Actually in Horsetail Extract
The plant contains more than 10% inorganic substances, roughly two-thirds of which are silicic acid (a form of silicon) and potassium salts. Silicon is a trace mineral your body uses in small amounts to support the structural proteins in skin, hair, nails, and bones. Most people don’t think about silicon the way they think about calcium or iron, but it plays a supporting role in collagen formation.
Beyond silicon, horsetail is rich in plant-based antioxidants called flavonoids. The dominant ones in European-grown plants include compounds related to quercetin, apigenin, and kaempferol. Interestingly, horsetail grown in Asia and North America has a slightly different chemical profile, containing flavonoids not found in the European varieties. The extract also contains caffeic acid derivatives, which contribute additional antioxidant activity. These compounds collectively give horsetail its reputation as more than just a silicon supplement.
Diuretic Effect
One of the best-studied uses of horsetail extract is as a natural diuretic, meaning it helps your body shed excess fluid through increased urination. A randomized, double-blind clinical trial tested 900 mg per day of a standardized horsetail extract against hydrochlorothiazide (a commonly prescribed pharmaceutical diuretic) and a placebo in 36 healthy men over four-day cycles.
The results were notable. Participants taking horsetail lost an average of about 322 mL more fluid than they took in, compared to about 232 mL for those on the pharmaceutical diuretic. Statistically, the two were equivalent, with no significant difference between them. Both performed significantly better than placebo. Perhaps more importantly, horsetail did not alter sodium or potassium excretion rates, which is a common concern with pharmaceutical diuretics that can deplete electrolytes.
Skin, Hair, and Nail Uses
Horsetail shows up in a surprising number of beauty and skincare products, and there’s some biological basis for this. Animal research has found that topical application of horsetail extract increased collagen production by stimulating fibroblasts, the cells responsible for building the structural framework of skin. In diabetic rats with impaired wound healing, the extract produced a denser, more compact collagen layer in healed skin compared to untreated wounds. Several compounds in the extract, including quercetin and plant sterols, appear to promote both collagen synthesis and the migration of repair cells into damaged tissue.
For hair, one clinical trial found that women with self-perceived thinning hair who took capsules containing dried horsetail (along with other ingredients) for three months experienced increased hair growth and strength compared to a control group. The results are encouraging but hard to attribute solely to horsetail since the capsules contained multiple ingredients.
Horsetail extract has also been incorporated into nail lacquers designed to strengthen brittle nails. One study found that a nail polish formulated with horsetail extract and other hardening agents reduced visible signs of nail psoriasis, a condition that causes pitting, discoloration, and crumbling of the nails.
Standard Dosage
The most commonly studied dose is 900 mg per day of a dry extract, typically split into three 300 mg doses. This was the amount used in the diuretic trial and represents the maximum recommended daily dose for standardized dry extracts. Horsetail tea is also widely available, though standardized dosing is harder to control with loose-leaf preparations since the concentration of active compounds varies with steeping time and plant quality.
Safety Concerns
Horsetail extract carries a few specific risks worth understanding before you take it.
The first involves an enzyme called thiaminase, which breaks down thiamine (vitamin B1) in the body. Raw or minimally processed horsetail contains this enzyme, and prolonged consumption can lead to thiamine deficiency. This is well documented in horses that graze on horsetail plants, but it’s a relevant concern for humans using crude preparations over long periods. Commercially produced extracts are generally processed to reduce thiaminase activity, though the specific methods vary by manufacturer.
The second concern is species confusion. The medicinal species, Equisetum arvense, must be carefully distinguished from Equisetum palustre (marsh horsetail), which contains toxic alkaloids called palustrine and palustridiene. Contamination of horsetail products with the wrong species is a documented problem, and sophisticated lab techniques are needed to reliably detect adulteration. Buying from reputable supplement brands that test for species identity reduces this risk.
Drug Interactions
Because horsetail is an effective diuretic, it can interact dangerously with lithium, a medication used for bipolar disorder. Diuretics reduce the body’s ability to clear lithium, causing blood levels to climb into toxic ranges. A case report published in the American Journal of Psychiatry described lithium toxicity in a patient taking a herbal supplement containing horsetail alongside other diuretic herbs. The interaction was significant enough that the patient developed clinical toxicity.
For the same reason, combining horsetail with prescription diuretics could amplify fluid and electrolyte loss beyond what’s intended. People taking medications for blood pressure, diabetes, or any condition affected by fluid balance should be cautious about adding horsetail extract without professional guidance.
Regulatory Status
Horsetail extract is sold as a dietary supplement in the United States, not as an approved drug. The FDA lists Equisetum arvense extract with a “non-approved” status, meaning it has not undergone the formal review process required for pharmaceutical products. This is standard for herbal supplements, which are regulated under the Dietary Supplement Health and Education Act rather than the drug approval pathway. The practical consequence is that quality, potency, and purity can vary between brands, making third-party testing certifications a useful thing to look for on the label.