Herb-Drug Interactions: Horsetail

2011

Equisetum arvense L. (Equisetaceae)

Synonym(s) and related species

Equisetum.

The related species Equisetum hyemale L. has also been used, but note that standardised pharmacopoeial preparations of horsetail should contain no more than 5% of other Equisetum species.

Pharmacopoeias

Equisetum Stem (Ph Eur 6.04); Horsetail (British Pharmacopoeia 2009).

Constituents

Horsetail contains high concentrations of silicic acid, up to 8%, and is sometimes used as an organic source of silicon. It also contains flavonoids such as apigenin, kaempferol, luteolin and quercetin and their derivatives, and may be standardised to the total flavonoid content expressed as isoquercitroside. Other polyphenolic compounds such as caffeic acid derivatives, and trace amounts of the alkaloid nicotine, and sterols including cholesterol, isofucosterol and campesterol, are also present. Horsetail also contains thiaminase (an enzyme that breaks down thiamine), and this is inactivated in some supplements.

Use and indications

Horsetail is used mainly as an astringent, haemostatic and anti-inflammatory agent, and for urinary tract complaints such as cystitis, prostatitis, urethritis and enuresis. There is little pharmacological, and no clinical, evidence to support the main uses.

Pharmacokinetics

An in vitro study using alcoholic extracts of horsetail found that it had only weak inhibitory effects on the cytochrome P450 isoenzyme CYP3A4. For information on the pharmacokinetics of individual flavonoids present in horsetail, see under flavonoids.

Interactions overview

An isolated case of lithium toxicity has been reported in a patient who took a herbal diuretic containing horsetail among other ingredients, see under Parsley + Lithium. For information on the interactions of individual flavonoids present in horsetail, see under flavonoids.

Horsetail + Food

No interactions found.

Horsetail + Herbal medicines

No interactions found.

Horsetail + Lithium

‘For mention of a case of lithium toxicity in a woman who had been taking a non-prescription herbal diuretic containing corn silk, Equisetum hyemale, juniper, buchu, parsley and bearberry, all of which are believed to have diuretic actions, see under Parsley + Lithium. Note that this case was with Equisetum hyemale, which is not the species more commonly used (Equisetum arvense).