Herb & supplement monograph

Horsetail Drug Interactions, Uses, Effectiveness, Safety & More

Medically reviewed by Dr. Brian Staiger, PharmD, BCPS ·Updated July 2026

What is this page for?

First and foremost: how Horsetail interacts with medications. The heart of this page is the interaction list — every drug Horsetail is known to interact with, and how serious each one is.

But these pages have grown well beyond that into a full monograph — what Horsetail is, what people use it for and how strong the evidence is, its safety and side effects, and answers to the questions we’re asked most — written and reviewed by the clinical staff at HelloPharmacist. It’s educational information from our licensed clinical databases, not medical advice, and we don’t sell or endorse products. Our editorial policy

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Horsetail Drug Interactions: The Bottom Line

From the HelloPharmacist Editorial Team · Updated July 2026

Based on the available evidence, the overall risk of a clinically significant Horsetail drug interaction appears low for most people, though a few medicine groups deserve care. None of the listed interactions are rated major, and nearly all rest on theoretical or limited evidence rather than well-documented human studies.

Interactions deserving the most attention

The strongest signal involves HIV medications, including efavirenz and NRTI drugs, where case reports describe patients whose virus became detectable again while using horsetail-containing supplements. Lithium also warrants care, since horsetail's possible water-losing effect could, in theory, raise lithium levels, and lithium has a narrow safety margin. There is also a theoretical chance that horsetail could lower blood sugar when combined with diabetes medications, or add to potassium loss with certain diuretics.

What the rest of the list means

A long list of interacting medications does not mean each combination is harmful. Most entries are predictions based on how horsetail might act in the body, drawn from lab or animal research rather than studies in people. Notably, human research on the diuretic concern has not confirmed the predicted effect, so many of these warnings reflect caution rather than proven problems.

Check your medications against Horsetail

Based on HelloPharmacist’s Horsetail interaction data and reviewed under our editorial standards.

Interaction report

Drugs that interact with Horsetail

188 medications have a known interaction with Horsetail, graded by severity. Select any drug for the full evidence-based detail.

Don’t want to scroll the list? Just ask.

Tell us the medications you take and we’ll check each one against Horsetail using our pharmacist-reviewed interaction data.

AI summaries are generated from our Horsetail interaction data for education only — always confirm with your pharmacist. How we use AI

2,830 drugs

Read The List Like a Pharmacist

What Severity, Likelihood & Evidence Mean

Severity — How Serious It Can Be

Major. Clinically significant; generally best avoided, or used only under direct professional supervision.
Moderate. May need monitoring, a dose adjustment, or separating the times you take each one.
Minor. Generally not clinically significant, but still worth noting and mentioning to your pharmacist.
No known interaction. Checked against our sources with nothing documented — not the same as proven safety.

Likelihood — How Well It’s Documented

Likely. Well-controlled human studies have demonstrated the likely existence of this interaction
Probable. Interaction has not been documented in well-controlled studies, however, the interaction has been demonstrated in some small human studies or in controlled animal studies in conjunction with multiple case reports.
Possible. Interaction has been documented in animal or in lab research, or the interaction has been documented in humans but is limited to case reports or conflicting clinical research exists
Unlikely. Interaction has been demonstrated in animal or in lab research but has been shown not to occur in humans.

Where This Data Comes From

Interaction records are evidence-graded and sourced from the Natural Medicines database (TRC Healthcare), the same reference used by pharmacists and hospitals.
Each drug listed above links to the full report for that exact Horsetail combination — clinical detail, likelihood, evidence level, and citations.
Content is reviewed by licensed HelloPharmacist pharmacists — see our data sources and editorial standards.

The big picture

The kinds of drugs Horsetail affects

Every type of medication (drug category) Horsetail is known to interact with. Open any category for the detail — or search your exact drug in the checker above.

All 5 drug categories Horsetail interacts with

Antidiabetes DrugsDiuretic DrugsEfavirenz (sustiva)LithiumNucleoside Reverse Transcriptase Inhibitors (nrtis)

Antidiabetes Drugs

Theoretically, taking horsetail with antidiabetes drugs might increase the risk of hypoglycemia.
Equisetum myriochaetum has demonstrated hypoglycemic activity in clinical research. In an animal diabetic model, Equisetum giganteum had hypoglycemic effects. It is unclear whether other horsetail species have hypoglycemic effects.

Likelihood Possible Evidence D

Diuretic Drugs

Theoretically, taking horsetail with diuretic drugs might increase potassium loss and the risk of hypokalemia.
Laboratory research shows that various species of horsetail have diuretic properties. Due to its diuretic effects, there has been concern that taking horsetail along with potassium-depleting diuretics might increase the risk for hypokalemia. However, pharmacokinetic research in humans shows that taking horsetail 900 mg daily for 4 days does not affect urinary excretion of electrolytes, including potassium and sodium, despite having a diuretic effect similar to taking hydrochlorothiazide 25 mg daily. It is unclear if taking horsetail for a longer duration would affect electrolyte levels. Until more is known, use with caution.

Likelihood Possible Evidence D

Efavirenz (Sustiva)

Theoretically, horsetail might decrease the levels and clinical effects of efavirenz.
In two case reports, patients were found to have detectable viral loads when taking horsetail-containing supplements along with an antiretroviral regimen that included efavirenz. In one case, the antiretroviral regimen included zidovudine, lamivudine, and efavirenz; in the other case, the regimen consisted of emtricitabine, tenofovir disoproxil fumarate, and efavirenz. One month after discontinuing horsetail, the viral loads became undetectable in both cases. The exact mechanism of this interaction is unknown. It is also unclear if this interaction is specific to efavirenz or if it is related to various components of antiretroviral therapy.

Likelihood Possible Evidence D

Lithium

Theoretically, horsetail might increase the levels and adverse effects of lithium.
Animal research suggests that horsetail has diuretic properties. Theoretically, due to these potential diuretic effects, horsetail might reduce excretion and increase levels of lithium. The dose of lithium might need to be decreased.

Likelihood Possible Evidence D

Nucleoside Reverse Transcriptase Inhibitors (Nrtis)

Theoretically, horsetail might decrease the levels and clinical effects of NRTIs.
In two case reports, patients were found to have detectable viral loads when taking horsetail-containing supplements along with an antiretroviral therapy. In one case, the antiretroviral regimen included zidovudine, lamivudine, and efavirenz; in the other case, the regimen consisted of emtricitabine, tenofovir disoproxil fumarate, and efavirenz. One month after discontinuing the supplement, the viral loads became undetectable in both cases. The exact mechanism of these interactions is unknown. It is also unclear if these interactions are specific to NRTIs or if they are related to various components of antiretroviral therapy.

Likelihood Possible Evidence D

Monograph

Horsetail: Uses, Safety & Side Effects

Medically reviewed by Dr. Brian Staiger, PharmD, BCPS · Reviewed July 2026 ·

The bottom line

Horsetail is a traditional herb most often used as a mild diuretic and for hair, nail, and bone support, but high-quality human evidence is limited. It can cause thiamine (vitamin B1) loss with long-term use, and some related species are toxic, so choose products carefully and talk to your pharmacist or doctor before using it.

Scientific name: Equisetum arvense
Family: Equisetaceae
Part used: Aerial parts (stems)
Common forms: Tea, capsules, tablets, liquid extract, tincture, dried herb

People commonly use it for

Fluid retention (mild diuretic)
Urinary and bladder concerns
Hair, skin, and nail support
Wound healing
Bone health

Popular and traditional uses — not proof it works. See “Uses & effectiveness” below for the evidence.

Safety at a glance

OverallUse caution

May be used short-term by healthy adults, but long-term use and unverified products carry real risks.

PregnancyInsufficient reliable information

Insufficient reliable information available; avoid using.

Read the full pregnancy detail

BreastfeedingInsufficient reliable information

Insufficient reliable information available; avoid using.

Read the full breastfeeding detail

Pregnancy & breastfeeding ratings are from Natural Medicines (Therapeutic Research Center). Safety guidance is general; always confirm with your pharmacist or doctor for your situation.

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Overview

Horsetail (Equisetum arvense) is a perennial plant with hollow, jointed, bristle-like stems that look a bit like a horse's tail. It grows wild across much of the Northern Hemisphere, often in damp soil, ditches, and along roadsides.

People have used horsetail in traditional medicine for centuries. It is most often taken as a tea or in capsules. Common traditional uses include acting as a 'water pill' (diuretic) for swelling, supporting urinary and bladder health, and promoting healthy hair, skin, nails, and bones.

It is important to know that not all horsetail is the same. Equisetum arvense is the species used in supplements, while a related species, Equisetum palustre (marsh horsetail), can be toxic. Quality products should clearly identify the correct species.

How it works

Horsetail contains several compounds that may explain its traditional uses, though most of this comes from laboratory and animal research rather than strong human studies.

It is high in silica (silicon), which the body uses in connective tissue, and this is the basis for claims about hair, nails, and bones. It also contains flavonoids and other plant antioxidants that may have anti-inflammatory effects in test-tube studies.

Its mild diuretic effect is thought to come from these plant compounds prompting the kidneys to release more water and salt. Horsetail also contains an enzyme called thiaminase, which can break down vitamin B1 (thiamine) — this is a safety concern rather than a benefit.

Effectiveness

Does Horsetail work?

Here’s what the research actually shows for each use — graded by Natural Medicines and reviewed by Dr. Brian Staiger, PharmD.

Evidence overview · 9 uses evaluated

9 Insufficient evidence

How to read these evidence grades

Natural Medicines’ 7-point scale. We show each rating’s label word-for-word.

1EffectiveStrong, consistent evidence it works.

2Likely EffectiveGood evidence, though not yet conclusive.

3Possibly EffectiveSome evidence suggests a benefit.

4Possibly IneffectiveSome evidence it may not help.

5Likely IneffectiveFairly strong evidence it doesn’t help.

6IneffectiveStrong evidence it doesn’t work.

7Insufficient Reliable Evidence to RateToo little research to say either way.

For Horsetail, current evidence isn’t strong enough to rate any specific use. The conditions it has been studied for are listed below.

Also studied for 9 conditions — evidence insufficient to rate

Insufficient Reliable Evidence To Rate Aging skin