Horsetail Safety: Thiaminase, Nicotine and Dose Limits

This is the page that supplement marketing does not write. Horsetail is a genuinely interesting traditional herb, and it also carries four safety features that are not optional reading: an enzyme that destroys vitamin B1, a toxic look-alike species that has turned up in commercial material, trace nicotine, and a real diuretic effect with the drug interactions that any diuretic brings.

None of that makes horsetail dangerous in the way a poisonous plant is dangerous. Most people who drink horsetail tea for a few weeks will be fine. What it does mean is that horsetail is one of the herbs where duration of use is a genuine safety parameter, where the label matters, and where a specific group of people should not take it at all. The goal here is to explain the mechanisms honestly — not to bury them and not to inflate them — so that the decision is yours and it is informed.

Why This Page Exists
Thiaminase: What the Enzyme Does
The Veterinary Evidence: Horses and Cattle
What Thiamine Deficiency Looks Like in People
How Real Is the Human Risk? An Honest Calibration
Heat, Processing and “Thiaminase-Free” Labels
The Double Hit: A Thiaminase-Containing Diuretic
Species Identity: Equisetum palustre and Palustrine
Trace Nicotine and Other Alkaloids
Selenium, Heavy Metals and Where It Grew
Drug Interactions
Duration of Use: Why There Is a Cap
Who Should Avoid Horsetail Entirely
A Practical Checklist for Buying and Using
Warning Signs and What to Do
Bottom Line
Key Research Papers
Connections

Why This Page Exists

Search for horsetail and you will find hundreds of pages describing it as a silica-rich beauty herb, and a striking number that mention no cautions whatsoever. Meanwhile the European regulatory assessment of horsetail restricts how long it may be used, and the reason is a specific biochemical property of the plant. When a regulator caps duration and a marketer does not mention the reason, the reader is missing the single most decision-relevant fact.

There is a second reason. Horsetail's cautions are unusually mechanistic — they are not vague “may cause side effects in some individuals” boilerplate. Thiaminase does a defined chemical thing to a defined vitamin. Palustrine is a defined alkaloid in a defined species. A diuretic effect has defined pharmacokinetic consequences for lithium. Mechanisms of that kind can be reasoned about, which means a reader can make a genuinely informed decision rather than an anxious guess. That is what the rest of this page is for.

Thiaminase: What the Enzyme Does

Thiamine, vitamin B1, is a small molecule with two ring systems joined by a methylene bridge: a pyrimidine ring and a thiazole ring. In the body it is phosphorylated to thiamine pyrophosphate, which is an essential cofactor for several enzymes that sit at the centre of energy metabolism — pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase, the branched-chain ketoacid dehydrogenase, and transketolase in the pentose phosphate pathway. Without it, cells cannot fully oxidise glucose. Tissues that run hardest on glucose — brain and heart — fail first.

Thiaminase is an enzyme that cleaves that bridge, splitting thiamine into two fragments that have no vitamin activity. There are two families. Thiaminase I performs a base-exchange reaction, substituting another molecule for the thiazole ring; thiaminase II simply hydrolyses the bond. Either way the vitamin is destroyed.

The important consequences of thiaminase being an enzyme rather than a toxin:

It is catalytic. A small quantity can degrade a much larger quantity of thiamine, so the effect is not proportional to a “dose of poison” in the ordinary sense.
It acts on thiamine, not on you. The harm is indirect: nutrient depletion over time, not acute toxicity. That is why the exposure that matters is prolonged rather than large-and-single.
It is a protein, so it is heat-labile. Cooking, boiling and many drying and extraction processes denature it. This is the basis of “thiaminase-free” processed horsetail products.
It can destroy thiamine in the food or gut rather than in tissues. Thiaminase-containing material eaten alongside thiamine-containing food can degrade that dietary thiamine before it is absorbed.

Thiaminase activity in Equisetum is long established, and it is the documented reason European herbal guidance limits how long horsetail may be taken. Tier: established biochemistry. PubMed search: Equisetum thiaminase thiamine degradation PubMed search: thiaminase mechanism of thiamine cleavage

The Veterinary Evidence: Horses and Cattle

Everything solid we know about thiaminase-driven deficiency from plants comes from animals, and it is worth understanding why the animal case is so much stronger than the human one.

Horses are the classic case. Horses that consume large amounts of horsetail — usually because it has been baled into hay, where they cannot avoid it and where its unpalatability no longer protects them — can develop signs of thiamine deficiency: weight loss, weakness, incoordination and ataxia, and in advanced cases collapse. The syndrome responds to thiamine administration, which is the crucial piece of evidence: a deficiency that reverses with the vitamin is a deficiency, not a general poisoning. The parallel and better-studied case is bracken fern (Pteridium aquilinum), whose thiaminase produces the same picture in horses and is a well-described veterinary poisoning.

Cattle and sheep are affected differently. Ruminants make some thiamine in the rumen, which buffers them, so classic bracken or horsetail thiaminase disease is less prominent in cattle than in horses — though ruminant thiamine deficiency does occur, and horsetail-contaminated forage has been implicated in reduced condition and in poisoning reports. Cattle problems with Equisetum are also frequently a different plant problem: Equisetum palustre, the toxic species, whose alkaloids are a separate mechanism entirely.

The reason animals get the disease and people generally do not is dose and duration. A horse eating contaminated hay may consume kilograms of dry plant material a day for months. A person drinking two cups of horsetail tea a day is consuming grams. That is a difference of three orders of magnitude, sustained over a much longer period, in an animal with a different digestive system. Saying this plainly is not dismissing the risk — it is sizing it. Tier: veterinary case and toxicology literature. PubMed search: Equisetum poisoning in horses PubMed search: bracken fern thiaminase in horses

What Thiamine Deficiency Looks Like in People

Knowing the clinical picture is what turns an abstract caution into something you could actually notice.

Early and non-specific: fatigue out of proportion to activity, loss of appetite, irritability, poor concentration, unrefreshing sleep, vague muscle discomfort, mild calf tenderness. These overlap with a hundred other things, which is exactly why early thiamine deficiency is missed.

Dry beriberi is the nervous-system form: a symmetrical peripheral neuropathy beginning in the feet with burning, numbness and pins and needles, ascending over time, with weakness, loss of reflexes and difficulty walking. See dry beriberi and the nerves.

Wet beriberi is the cardiovascular form: a high-output heart failure with breathlessness, rapid heart rate, swelling of the legs, and in severe untreated cases circulatory collapse. See wet beriberi and the heart.

Wernicke encephalopathy is the acute brain emergency: confusion, abnormal eye movements and unsteady gait, which can progress to the permanent memory disorder of Korsakoff syndrome if thiamine is not given quickly. It is most associated with alcohol use disorder but occurs in any severe thiamine depletion. See Wernicke-Korsakoff syndrome.

Two features of thiamine make this worth respecting. Body stores are small — on the order of weeks, not months — and the vitamin is water-soluble and readily lost in urine, so depletion can develop faster than for most vitamins. The good news is that the deficiency is cheaply and rapidly correctable if recognised. See vitamin B1, thiamine deficiency and thiamine and beriberi. PubMed search: thiamine deficiency clinical review

How Real Is the Human Risk? An Honest Calibration

This section is the reason the page is worth reading, because both of the easy answers are wrong.

The overstated version — “horsetail causes vitamin B1 deficiency” — is not supported. There is no substantial published series of human beriberi caused by horsetail consumption. If drinking horsetail tea reliably produced thiamine deficiency, a herb this widely used would have generated a visible case literature, and it has not.

The dismissive version — “that only affects horses, ignore it” — is also wrong, for three reasons.

First, the mechanism is not in doubt. The enzyme exists, its substrate is a vitamin with small body stores, and its action is catalytic. A mechanism this clean does not require a case series to be taken seriously as a reason to limit duration.

Second, dietary thiaminase demonstrably matters in humans in other contexts. Thiaminase-containing raw and fermented fish has been implicated as a contributing factor in human thiamine deficiency in populations where such foods form a large and habitual part of the diet. That precedent establishes the principle: eaten in quantity, over time, dietary thiaminase can affect human thiamine status. Tier: human observational and analytical. PubMed search: thiaminase in raw and fermented fish and human thiamine deficiency

Third, risk is not uniform. A well-nourished person with full thiamine stores has considerable buffer. Someone whose stores are already thin has almost none. The people in that second category are not rare: heavy alcohol use, poor or very restricted intake, after bariatric surgery, with persistent vomiting or malabsorption, in advanced age with poor appetite, on long-term loop diuretics, and in poorly controlled diabetes. In those people a thiaminase-containing herb taken daily for months is a genuinely bad idea, and the fact that no case report exists is not reassurance — it is an absence of data about an obviously higher-risk group.

The honest formulation, then: horsetail thiaminase is a mechanistically established, clinically undocumented risk in humans, whose plausibility rises sharply with duration of use and with pre-existing thiamine marginality. That is exactly the risk profile that a duration limit is the correct response to — which is what European guidance does — and it is why any suggestion of long-term horsetail use should carry a B1 caution beside it. A useful, cheap mitigation, if someone chooses to take horsetail for more than a short course: take a thiamine-containing supplement or a B-complex at a different time of day, and keep the course finite. Separating them in time is the point, since thiaminase can degrade thiamine in the gut.

Heat, Processing and “Thiaminase-Free” Labels

Because thiaminase is a protein, it can be destroyed — and this is the most practically useful fact on the page.

Proteins denature with heat. Boiling the herb, or subjecting the raw material to a hot processing or extraction step, denatures thiaminase and abolishes its activity. Some manufacturers state that their product is thiaminase-free or has been treated to inactivate the enzyme, and that claim is chemically coherent rather than marketing invention.

Reasoning forward from that, with the appropriate label:

A boiled decoction — simmered, not merely steeped in off-the-boil water — plausibly contains little or no active thiaminase, because the enzyme has been cooked. Tier: mechanistic inference from protein chemistry, not a measurement of horsetail preparations.
A cold infusion or cold macerate preserves the enzyme, and is therefore the least reassuring preparation.
Dry powdered herb in capsules is uncertain. Whether the enzyme survives depends entirely on how the material was dried and processed; low-temperature drying can preserve enzyme activity. A capsule is also swallowed with no heating step at all.
A hot-water or hot-ethanol extract has had a heating step; a cold alcohol tincture has not, though the alcohol itself will denature protein to some degree.

Two honest limits on this reasoning. It has not, as far as the published literature goes, been settled by measuring thiaminase activity across real commercial horsetail preparations — so a “thiaminase-free” claim is only as good as the manufacturer making it. And inactivating the enzyme does nothing about the other three issues: species identity, nicotine and the diuretic effect are unaffected by heat. A “thiaminase-free” label is not a general safety certificate. PubMed search: thiaminase heat inactivation

The Double Hit: A Thiaminase-Containing Diuretic

Here is an interaction internal to the plant itself, and it is rarely pointed out.

Thiamine is water-soluble and excreted in urine. Increasing urine output increases urinary thiamine loss. This is not speculation about horsetail; it is a documented phenomenon with pharmaceutical diuretics. Thiamine deficiency has been repeatedly described in patients on long-term high-dose loop diuretic therapy for heart failure, and it is attributed in large part to increased urinary losses; some studies of thiamine supplementation in that population have reported improvements in cardiac measures. Tier: human observational, with small interventional studies. PubMed search: loop diuretics, thiamine deficiency and heart failure

Horsetail therefore does both things at once: it contains an enzyme that destroys thiamine, and it produces a diuresis that excretes thiamine. Neither effect alone is likely to matter over a couple of weeks in a well-nourished person. Together, sustained over months, in someone whose reserves are already marginal, they push in the same direction.

This convergence is the strongest single argument for the thing regulators already say: keep the course short. It is also a specific reason that combining horsetail with a prescribed loop diuretic is a poor idea for a reason beyond the obvious over-diuresis — the person on furosemide is already the person at risk of losing thiamine in their urine.

Species Identity: Equisetum palustre and Palustrine

This is the fastest way for horsetail to cause real harm, and it has nothing to do with the medicinal plant.

Equisetum arvense (field or common horsetail) is the medicinal species. Equisetum palustre (marsh horsetail) is a close relative that contains the piperidine alkaloid palustrine and related compounds, is regarded as toxic to livestock, and has been implicated in animal poisonings with neuromuscular signs. The two species grow in overlapping habitats — damp ground, ditches, wet meadows — and distinguishing them requires attention to stem, sheath and branch characters that a non-botanist will not reliably apply, especially to dried, cut material where the diagnostic features are gone.

Two consequences follow.

Do not wild-harvest horsetail unless you can positively identify the species. This is not general herbal caution; it is a specific one, because the wrong plant in this genus contains an alkaloid rather than merely being useless.

Commercial adulteration and admixture are documented problems. Analytical methods exist specifically to discriminate E. arvense from E. palustre in herbal drug material — chromatographic fingerprinting and flavonoid-profile comparison — and methods of that kind are developed because the problem is real, not hypothetical. Pharmacopoeial monographs for Equiseti herba include identity tests for the same reason. In practice this means: a product whose label says only “horsetail” is unidentified plant material, and a supplier who cannot tell you the species and how they verified it is not a supplier for this herb. Tier: established toxicology plus analytical chemistry. PubMed search: Equisetum palustre palustrine toxicity PubMed search: distinguishing E. arvense from E. palustre in herbal material

Trace Nicotine and Other Alkaloids

Equisetum arvense contains small amounts of nicotine and related alkaloids. This should be neither ignored nor dramatised.

Why it is not dramatic: the quantities reported are trace, far below what a cigarette or a nicotine product delivers, and there is no suggestion that ordinary horsetail tea produces nicotine effects or dependence in an adult.

Why it still matters:

Children. Nicotine's toxicity is dose-per-body-weight, and small children are disproportionately sensitive. Combined with the absence of any safety data in children, this is a firm reason not to give horsetail to a child.
Pregnancy and breastfeeding. Nicotine crosses the placenta and enters breast milk, and no amount has been established as safe in pregnancy. Horsetail is not recommended in either state — the alkaloid content is one reason among several.
Very high or prolonged dosing. Trace amounts of an alkaloid taken in large quantity for a long time is a different exposure from trace amounts taken briefly, and it is another argument against open-ended use.

Reported alkaloid content varies with species, growing conditions and analytical method, and figures in the literature are not uniform — another reason to treat any single number quoted online with caution. Tier: analytical chemistry. PubMed search: Equisetum arvense nicotine and alkaloid content

Selenium, Heavy Metals and Where It Grew

Horsetail is an efficient mineral accumulator — that is the same property that makes it silica-rich — and accumulation is not selective for the mineral you want. Equisetum species have been used in biomonitoring and phytoremediation studies precisely because they take up and concentrate elements from soil and water, including heavy metals.

Two practical implications:

Selenium. Horsetail growing on selenium-rich ground can concentrate it. Selenium is essential in microgram quantities and toxic in excess, with chronic excess causing hair and nail loss, brittle nails, garlic-smelling breath, gastrointestinal upset and neurological symptoms — an unpleasant irony for a herb sold for hair and nails. See selenium.
Heavy metals. Roadsides, industrial margins, ditches beside farmland and drainage channels are exactly where horsetail thrives and exactly where contamination is likely. Cadmium and lead uptake by wild plants from such sites is well described.

This is the strongest argument for buying tested commercial material rather than foraging, even for someone confident of the species. A supplier who tests for heavy metals and for identity is providing something you cannot replicate with a pair of scissors and a ditch. Tier: environmental analytical chemistry. PubMed search: Equisetum heavy metal accumulation

Drug Interactions

These follow mostly from the diuretic effect and are covered in more detail in the diuretic article. In order of importance:

Lithium — do not combine. Diuresis and volume depletion reduce renal lithium clearance and can raise blood levels into the toxic range. Lithium has a narrow therapeutic window and toxicity is a medical emergency.
Prescription diuretics — do not stack. Loop, thiazide and potassium-sparing agents are dosed deliberately; adding an unmeasured herbal diuretic risks volume depletion, hypotension and electrolyte disturbance. For loop diuretics there is the additional thiamine argument above.
Digoxin. Toxicity is potassium-dependent; anything that might lower potassium raises the risk of arrhythmia.
Antihypertensives of all classes — additive blood-pressure lowering, dizziness, and impaired kidney function if volume-depleted.
Non-steroidal anti-inflammatory drugs — combined with a diuretic and a renin-angiotensin blocker, a recognised route to acute kidney injury.
Anything that lowers thiamine or potassium independently — heavy alcohol use, corticosteroids, stimulant laxatives.
Nicotine-containing products. The contribution from horsetail is trace, but there is no reason to add an unmeasured source while using patches or gum.
Diabetes medication. Some sources raise the possibility of blood-glucose effects; the evidence is weak, but anyone monitoring glucose closely should watch their readings when starting any new herb.

Duration of Use: Why There Is a Cap

Most herbal safety advice is about dose. For horsetail, the parameter that actually carries the risk is time, and this is the single most useful reframing on the page.

Every one of the four hazards scales with duration rather than with a single dose. Thiaminase depletes a vitamin with weeks of body stores, so the exposure that matters is cumulative. Trace alkaloids taken briefly are trivial and taken for a year are a different exposure. Diuresis is a physiological state that is fine transiently and not a good permanent condition. Mineral accumulation from contaminated material is, again, cumulative.

This is why European traditional-use guidance for horsetail restricts the duration of use rather than merely stating a dose, and why the practical rule is: a course with an end date, measured in weeks, followed by a break — not a bottle you re-order indefinitely.

It also produces the honest tension noted in the hair, skin and nails article: nails take four to six months to replace and hair density takes six to twelve months to shift, so a duration-limited herb cannot plausibly be assessed against those endpoints. If someone genuinely wants a months-long silicon trial, the right choice is a well-absorbed silicon supplement without thiaminase in it — see silicon — not a longer course of horsetail.

Who Should Avoid Horsetail Entirely

Pregnant women. No established safety, plus nicotine and alkaloid content, plus a diuretic effect in a state where fluid balance changes physiologically. Avoid.
Breastfeeding women. Same reasoning; nicotine passes into milk.
Infants and children. No safety data, and nicotine toxicity is weight-dependent. Avoid.
Anyone with low or borderline thiamine status. Heavy alcohol use is the largest group, and the most important: alcohol both reduces thiamine intake and impairs its absorption and activation, so alcohol plus a thiaminase-containing herb is the one combination with a genuinely predictable harm. Also relevant after bariatric surgery, with persistent vomiting, hyperemesis, malabsorption, prolonged poor intake, refeeding after starvation, and in advanced age with poor appetite.
Anyone taking lithium.
Anyone taking prescription diuretics or digoxin.
Anyone with kidney disease or heart failure. Fluid and electrolyte balance in these conditions is actively managed; Commission E contraindicated irrigation therapy in cardiac or renal insufficiency for exactly this reason. See kidney disease.
Anyone with a nicotine sensitivity or who is deliberately avoiding all nicotine exposure.
Anyone who cannot confirm the species on the label.
Tested athletes — diuretics are prohibited in sport both directly and as masking agents.

A Practical Checklist for Buying and Using

Species on the label. It must say Equisetum arvense. No species, no purchase.
Identity and contaminant testing. Ask, or buy from a supplier who publishes it. Heavy metals and species verification are the two that matter here.
Thiaminase status. Prefer a product that states the enzyme has been inactivated, or use a boiled decoction rather than a cold preparation. Treat a “thiaminase-free” claim as a manufacturer's assertion, not a guarantee.
Set an end date before you start. Weeks, not indefinitely. Write it down.
Check your medication list against the interaction list above, and ask a pharmacist — pharmacists are good at this and it costs nothing.
Drink water. If you are taking it as a diuretic, generous fluid intake is part of the traditional method, not an optional extra.
Consider a B-complex or thiamine supplement, taken at a different time of day, if the course will run more than a couple of weeks. Cheap, harmless, and directly aimed at the one mechanism.
Do not give it to children, and keep it where children cannot get it.
Do not forage it unless you can positively identify the species, and never from roadsides, drainage ditches or industrial margins.
Stop if anything on the warning list below appears.

Warning Signs and What to Do

Stop the herb and seek medical advice if you notice:

Burning, numbness or pins and needles in the feet or hands, or new unsteadiness on your feet — the pattern of thiamine-related neuropathy.
Unusual fatigue, appetite loss, irritability or poor concentration developing during a prolonged course.
Confusion, double vision or abnormal eye movements with unsteadiness — this triad is a medical emergency and thiamine is given urgently.
Breathlessness, rapid heartbeat, palpitations or swelling of the legs.
Muscle cramps or weakness — consider potassium.
Dizziness on standing, dark scanty urine, marked thirst — volume depletion.
Hair loss, brittle nails, a garlic-like breath odour or gastrointestinal upset — the picture of selenium excess, and a reason to question the material's provenance.
Any new rash, or any symptom that started when you started the herb.

If thiamine deficiency is a genuine concern, it is worth knowing that treatment is fast, cheap and safe, and that clinicians give thiamine on suspicion rather than waiting for a test result, because the test is slow and the untreated condition is not. Mentioning the herb to the clinician matters — a supplement history is frequently not asked for and is exactly the sort of detail that resolves a puzzling case.

Bottom Line

Horsetail is a legitimate traditional herb with one genuine, trial-supported action — diuresis — and four safety features that follow it around. Its thiaminase destroys vitamin B1 by a mechanism that is beyond dispute and a clinical consequence that is well documented in horses and essentially undocumented in people; the correct response to that combination is a duration limit rather than either alarm or silence. Its diuretic effect adds urinary thiamine loss to enzymatic thiamine destruction, which is why the duration limit is not arbitrary. Its toxic look-alike, Equisetum palustre, makes species verification a genuine safety step rather than a botanical nicety. Its trace nicotine and its mineral-accumulating habit are real if minor, and both argue in the same direction: short courses, tested material, not for children, not in pregnancy.

Used that way — correctly identified Equisetum arvense, from a tested source, ideally with the enzyme inactivated, for a course of weeks with an end date, with plenty of water, and not at all if you are pregnant, breastfeeding, a child, thiamine-marginal, on lithium, digoxin or prescription diuretics, or living with kidney or heart disease — horsetail is a reasonable thing for an interested adult to try. Used as an open-ended daily beauty supplement from an unlabelled bag, it is the herb whose risks were designed to be missed.

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Key Research Papers

Cited as PubMed topic searches. Titles, journals and years are stated only where confident; otherwise the finding is described and the search will surface the relevant literature.

Thiaminase activity in Equisetum. The primary literature establishing that horsetail contains thiamine-degrading enzyme activity — the basis for every duration limit placed on this herb. Tier: established biochemistry. PubMed search
Thiaminase I and II: mechanism. How the two enzyme families cleave thiamine, and why a catalytic enzyme behaves differently from a dose-dependent toxin. Tier: established biochemistry. PubMed search
Equisetum poisoning in horses. Veterinary reports of thiamine-responsive ataxia, weakness and weight loss in horses fed horsetail-contaminated hay — the clearest demonstration that plant thiaminase can cause disease. Tier: veterinary case literature. PubMed search
Bracken fern thiaminase and equine thiamine deficiency. The better-studied parallel case, useful for calibrating how much plant material and how long an exposure it takes. Tier: veterinary toxicology. PubMed search
Thiaminase in raw and fermented fish and human thiamine status. The human precedent showing dietary thiaminase can contribute to deficiency when consumed habitually and in quantity. Tier: human observational. PubMed search
Thiamine deficiency: clinical syndromes. Beriberi in its dry and wet forms, Wernicke encephalopathy and Korsakoff syndrome — what the risk actually looks like if it materialises. Tier: established clinical medicine. PubMed search
Loop diuretics and thiamine loss in heart failure. Documented urinary thiamine wasting with long-term diuretic therapy — the reason a thiaminase-containing diuretic is a double hit rather than a single one. Tier: human observational plus small intervention studies. PubMed search
Alcohol and thiamine status. Why heavy alcohol use is the group for whom this herb's thiaminase is not theoretical. Tier: established clinical medicine. PubMed search
Equisetum palustre and palustrine alkaloids. The toxicology of the look-alike species, and the livestock poisonings attributed to it. Tier: established toxicology, largely veterinary. PubMed search
Discriminating E. arvense from E. palustre in herbal drug material. Chromatographic and flavonoid-fingerprint methods developed because admixture and adulteration occur. Tier: analytical chemistry. PubMed search
Nicotine and alkaloid content of Equisetum arvense. Analytical reports of the trace alkaloid fraction. Tier: analytical chemistry. PubMed search
Mineral and heavy-metal accumulation in Equisetum. Biomonitoring and phytoremediation studies showing how efficiently the genus concentrates soil elements — the case for tested material over foraging. Tier: environmental analytical. PubMed search
Selenium excess: clinical features. Hair and nail loss, brittle nails and neurological symptoms — the ironic risk profile for a herb sold for hair and nails. Tier: established clinical medicine. PubMed search
Diuretics and lithium clearance. The interaction with the most serious consequences on this page. Tier: established pharmacokinetics. PubMed search
Carneiro DM and colleagues. Randomized, Double-Blind Clinical Trial to Assess the Acute Diuretic Effect of Equisetum arvense (Field Horsetail) in Healthy Volunteers. Evidence-Based Complementary and Alternative Medicine, 2014. Included here because it is the evidence that the diuretic effect — and therefore the interaction profile — is real. Tier: randomized clinical trial, acute. PubMed search
Herb-drug interactions and supplement history taking. Why telling the clinician about the herb changes outcomes, and how often the question is not asked. Tier: clinical reviews. PubMed search

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Connections

Safety and disclaimer. This page is educational and is not medical advice, and it is not a complete list of every possible risk or interaction. Horsetail contains thiaminase, which degrades vitamin B1, along with trace nicotine, and it has a toxic look-alike species, Equisetum palustre. Use only correctly identified Equisetum arvense from a supplier that tests identity and contaminants, in a course of weeks with a defined end date. Do not use it in pregnancy or breastfeeding, in children, with low or borderline thiamine status including heavy alcohol use, in kidney disease or heart failure, or alongside lithium, digoxin or prescription diuretics. If you develop numbness or tingling in the feet, new unsteadiness, confusion with abnormal eye movements, breathlessness, palpitations or leg swelling, stop the herb and seek medical attention promptly — and tell the clinician what you have been taking.

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