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Limited evidenceGeneral health

Horsetail Extract

Horsetail has a genuine, measurable diuretic effect — one small crossover trial found 900 mg/day matched hydrochlorothiazide 25 mg — but that is essentially the entire human evidence base. The hair, nail and bone claims that drive its sales have no supporting clinical trials at all.

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What the science says

  • Diuretic effect Limited evidence

    In a double-blind crossover trial, 36 healthy men took 900 mg/day of standardised Equisetum arvense extract for four days; the diuretic effect exceeded placebo and was equivalent to hydrochlorothiazide 25 mg/day without significant electrolyte loss. This is one small, short, single-centre study in healthy young men.

  • Blood pressure Limited evidence

    The same research group later published a double-blind randomised trial reporting an antihypertensive effect of E. arvense extract. It is a single small trial from one group and has not been independently replicated, so it should not guide treatment.

  • Hair, nails and bone density Limited evidence

    These are the claims horsetail is actually sold for, and they rest entirely on its silica content plus in vitro and animal data. There are no adequate randomised trials showing oral horsetail improves hair growth, nail strength or bone mineral density in humans.

Dosage & safety

Studied doseThe diuretic trial used 900 mg/day of standardised dried extract (300 mg three times daily) for four days. Traditional preparations use 2-3 g/day of dried herb as tea. No long-term dose has been established, and no dose has been validated for any cosmetic or bone-related use.
SafetyHorsetail contains thiaminase, an enzyme that destroys vitamin B1; prolonged use risks thiamine deficiency, which is well documented in grazing livestock and a genuine concern for chronic human use — look for products declared thiaminase-free. Some preparations contain trace nicotine. Its diuretic action means caution with lithium (levels can rise), other diuretics, and in kidney or heart disease, with attention to potassium. Avoid in pregnancy and breastfeeding, in children, and in people with alcohol use disorder, who are already thiamine-depleted. Buy only Equisetum arvense — the related marsh horsetail, Equisetum palustre, contains toxic alkaloids and is a known adulterant.

How to take it

TimingSplit across the day, and not late in the evening if the diuretic effect disturbs your sleep.
With food?With food and plenty of water.
Worth knowingConfirm the label says Equisetum arvense — the toxic look-alike Equisetum palustre is a documented adulterant of this ingredient.

Interactions

With medications

  • lithium

    The diuretic effect reduces lithium clearance and can raise levels into the toxic range. do not combine without lithium level monitoring

  • thiazide and loop diuretics

    Additive fluid and potassium loss. do not combine; have potassium checked if you use it at all

With other supplements

  • thiamine (vitamin B1)

    Horsetail contains thiaminase, an enzyme that destroys thiamine, so prolonged use risks deficiency. use only products declared thiaminase-free, keep courses short, and take a B1 supplement if using it repeatedly

Well-established interactions only — this is not a complete list. Always tell your doctor and pharmacist what you take.

Frequently asked questions

Does horsetail help hair and nail growth?

There are no adequate trials showing oral horsetail improves hair growth, nail strength or bone density. These claims rest on horsetail's silica content plus lab and animal data, so the evidence is limited.

Is horsetail a diuretic?

Yes — in one small trial of 36 healthy men, 900 mg/day of horsetail extract had a diuretic effect equivalent to hydrochlorothiazide 25 mg. That short study is essentially the entire human evidence base.

Is horsetail safe to take long term?

Horsetail contains thiaminase, which destroys vitamin B1, so prolonged use risks thiamine deficiency — use thiaminase-free products and keep courses short. Buy only Equisetum arvense, since the toxic marsh horsetail is a known adulterant.

Can horsetail be taken with lithium or diuretics?

Do not combine horsetail with lithium without level monitoring, as its diuretic effect can raise lithium into the toxic range. Avoid horsetail with thiazide and loop diuretics because of additive fluid and potassium loss.

Scientific references

Where to buy

Nature's Answer, Horsetail Extract, Alcohol-Free, 2,000 mg, 1 fl oz (30 ml)

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