Pennyroyal

Two similar plants go by the name pennyroyal, one native to Europe (and therefore called European pennyroyal) and one native to North America (and therefore called American pennyroyal). Both are members of the mint family (Lamiaceae) and grow in temperate regions of Europe and the Americas. The flowering tops are used as medicine, but the internal use of the volatile oil should be strictly avoided.

Since the time of the ancient Greeks, pennyroyal was considered a useful insect repellant, reflected in modern times by the common name fleabane.¹ The Latin names of both plants also reflect this insect-repelling power—pulegoides and pulegium both derive from the Latin word for flea. It was also believed to stimulate menstruation. Various folk herb traditions have employed American or European pennyroyal to help relieve coughs, upset stomachs, and anxiety.²

Like all mint family plants, pennyroyal owes much of its medicinal activity to the presence of a volatile oil. The primary component of this oil is known as pulegone. Pulegone is converted to menthofuran by the body. If large enough amounts of pulegone are consumed, the amount of menthofuran produced can seriously damage the liver and nervous system.³ Smaller amounts of the volatile oil contained in the whole plant appear to have mild, smooth, muscle-relaxing effects that might help explain the historical use of pennyroyal for indigestion, stomach cramps, and cough.⁴ No modern clinical trials have been completed to support these indications, and other herbs with soothing effects on the gastrointestinal tract, such as chamomile and peppermint, have a much greater history of safety than pennyroyal.

For adults (excluding pregnant or nursing women, children, and people with liver or kidney disease), a tea of pennyroyal can be prepared by putting 1–2 teaspoons (5–10 grams) of the herb in 1 cup (250 ml) of boiling water and allowing it to steep for 10–15 minutes.⁵ Up to 2 cups (500 ml) per day can be drunk. Pennyroyal tincture can be mixed with a skin cream and applied topically to repel insects, though it is unknown whether this is effective due to a lack of scientific study. The tincture and volatile oil are not recommended for internal use.

Used internally in the recommended amounts, pennyroyal is generally safe, though an occasional person may experience intestinal upset or temporary dizziness.⁶ Pulegone and its toxic metabolites, particularly menthofuran, damage the liver and nerves if taken in sufficiently large quantities.⁷ If used during pregnancy, pennyroyal may cause fetal death by liver and brain damage as well as promote uterine contractions to expel the fetus.⁸ Therefore pregnant or nursing women should absolutely avoid pennyroyal in any form. The traditional use of the herb to induce an abortion has led to many reports of nervous system toxicity in pregnant women. Internal ingestion of pennyroyal volatile oil should be avoided by everyone. People with liver failure or kidney failure, and all children, should avoid pennyroyal. Signs and symptoms of pennyroyal toxicity include severe stomach pain, dizziness, seizures, vomiting, difficulty walking, and coma. Since 1905, 18 cases of injury (with complete recovery in every case) and four deaths related to pennyroyal have been reported in the medical literature.⁹ The majority of acute poisonings and deaths reported with pennyroyal have been in cases of women using the oil attempting to induce an abortion.