Date of report 04 Jul 2024
Reported case interaction between
Efavirenz and Horsetail
Efavirenz and Horsetail
Drugs suspected to be involved in the DDI
Complete list of drugs taken by the patient
Horsetail (Equisetum arvense)
Clinical case description
A 49-year-old woman diagnosed with HIV in 2001 initiated antiretroviral therapy with zidovudine (ZDV), lamivudine (3TC), and nevirapine (NVP). In 2007, NVP was switched to efavirenz (EFV). Despite excellent adherence and sustained viral suppression, in 2013, she experienced two consecutive detectable viral loads (96 copies/mL and 57 copies/mL) over five months, while her CD4 count remained stable. Upon inquiry, she denied missed doses, immunizations, infectious diseases, and concomitant medications, but admitted to recently starting several herbal supplements. Specifically, she began taking daily horsetail (Equisetum arvense) supplements for its analgesic and anti-kidney stone effects two months before the first detectable viral load. Concerns regarding potential negative interactions between the herbal supplement and antiretroviral drugs led to the cessation of horsetail use. Subsequent viral load testing, one month post-discontinuation, showed resuppression to less than 50 copies/mL, with all subsequent viral loads remaining undetectable. The patient did not resume horsetail supplementation.
Equisetum arvense, known as horsetail, is mainly used for its diuretic properties. Equisetum is also used as analgesic, hemostatic, astringent, and for treatment of digestive disorders and kidney/bladder stones. Equisetum contains flavonoids and phenols among compounds that could induce CYP450. Moreover, due to its diuretic properties, Equisetum arvense may increase renal excretion of other drugs.
This case was published by Cordova E et al. in J Int Assoc Provid AIDS Care 2017.
Clinical Outcome
Drug Interaction Probability Scale (DIPS)
Editorial Comment
There are limited data about the pharmacological properties of Equisetum and its possible CYP450 metabolism. For these reasons, horsetail has been listed as an herb of undefined safety by the US Food and Drug Administration.
Equisetum contains flavonoids and phenols among compounds that could induce CYP450. Despite this, no studies have evaluated their role as potential CYP450 inducer. By contrary, a study found its role as a potential inhibitor of CYP1A2, CYP2D6, without significantly affecting CYP3A4.