Equisetaceae
Equisetum arvense
Horsetail · Field horsetail · Mare’s tail
- Part used
- aerial
- Native range
- Northern hemisphere throughout; naturalised and weedy elsewhere
- Morphology
- A living fossil — the last genus of a lineage that dominated Carboniferous forests as trees. Jointed hollow green stems with whorls of fine branches, reproducing by spores from separate pale fertile shoots, and stiff with silica.
What is in it
Evidence
Very little controlled evidence. A small trial supports a diuretic effect comparable to hydrochlorothiazide. Silica and bone or hair claims are unsupported.
Safety
Contains thiaminase, which destroys thiamine — vitamin B1 — and prolonged use can cause deficiency. This is well documented in horses poisoned by contaminated hay and is the basis of the human caution. Contaminating Equisetum palustre is genuinely toxic and hard to distinguish. Contraindicated in kidney or heart failure and in anyone with poor thiamine status, including in alcohol dependence.
Interactions: Additive with diuretics; Depletes thiamine; May increase lithium levels
Regulatory: Permitted as a listed ingredient; some jurisdictions require thiamine warnings.
How the traditions classify it
Three systems, rating the same plant. Where they disagree, none of them is correcting the others — they are separate frameworks that arrived at their categories independently.
Historical use
Used across Europe as a diuretic and styptic, and industrially as a scouring abrasive for pewter and wood — the silica content made it a household scrubber, which is why it is also called pewterwort.
Recorded as history and attributed where possible. Long use establishes that a plant was widely used, not that it worked, and not that it was safe.
Worth knowing
Horsetail is a genuine living fossil. Equisetum is the only surviving genus of the Equisetopsida, a class that formed tree-sized forests three hundred million years ago and whose remains are a meaningful fraction of the world's coal. The plant in a damp corner of an Australian garden is the last of something that once was a forest.
Its silica content made it a scouring pad long before it was a supplement, and that is the source of the modern marketing claim — silica for hair, skin and nails — which does not follow. Plant silica is not readily bioavailable, and no trial supports the claim.
The thiaminase is the reason it is not filed as low hazard. It destroys vitamin B1, the deficiency is well documented in horses, and prolonged human use in anyone with marginal thiamine status is a route to a preventable neurological injury.
Nomenclature verified against GBIF backbone taxonomy.