Pennyroyal

Mentha pulegium

Family: Lamiaceae Part used: Leaves and flowering tops (dried herb); NOT the essential oil

Key Compounds

• Pulegone
• Menthofuran
• Isomenthone
• Piperitone
• Rosmarinic acid
• Luteolin
• Apigenin
• Hesperidin
• Tannins
• Caffeic acid

Traditional Use

• Emmenagogue (menstrual stimulant) — pulegone stimulates uterine contractions; traditional European application for delayed or suppressed menstruation; this is the same mechanism as the abortifacient application, at lower doses and earlier in pregnancy; the distinction between emmenagogue use (delayed period restoration) and abortifacient use (pregnancy termination) is a dose and timing distinction, not a categorical one; contemporary herbalism restricts this to confirmed non-pregnant users with medically confirmed delayed menstruation under practitioner supervision
• Carminative and digestive — the volatile oils (pulegone, isomenthone) relax gastrointestinal smooth muscle, relieving gas, bloating, and digestive cramping; this application uses much lower concentrations than the emmenagogue application; the carminative use is the safest contemporary application and is dose-appropriate for short-course dilute infusion
• Insect repellent — pennyroyal's most commercially appropriate contemporary use; pulegone is a genuine and effective insect repellent, particularly against fleas; dried herb sachets repel fleas in household applications; the name 'pennyroyal' may derive from *pulex* (Latin, flea) through the form *puliall royall* (*pulegium regale*); topical preparations should use the herb, not the essential oil
• Diaphoretic (fever support) — traditional European application; the hot infusion promotes perspiration; standard combination with elderflower and yarrow for fever management; this application does not require the concentrations associated with reproductive applications and is dose-appropriate for the dried herb infusion

Pennyroyal essential oil has caused deaths. This is not a precautionary statement. It has happened.

In 1994, a woman in San Francisco died from pennyroyal essential oil. She had used it for abortion induction. This was not a new use — it is one of the oldest documented uses of the herb in European medicine, recorded since at least Hippocrates in 400 BCE and continuous through Dioscorides, Pliny, and medieval herbal texts. The dose made the difference. The essential oil contains pulegone at concentrations that cause hepatic necrosis. The herb in a cup of weak tea does not.

The European Medicines Agency reviewed pennyroyal and concluded the benefit-risk balance is unfavourable for any medicinal use.

The flea repellent works well and is considerably less eventful.

Meet the plant

A low-growing creeping mint with small, strongly scented leaves and whorls of lilac-purple flowers. The scent is mint-like but sharper and more camphoraceous than peppermint — unmistakable. Grows in damp grassland and stream margins across temperate Europe. The species pulegium connects directly to pulex (Latin, flea) — the flea-repellent property is in the name.

The toxicity mechanism

Pulegone constitutes up to 90% of pennyroyal essential oil. In the liver, pulegone is metabolised by cytochrome P450 enzymes (principally CYP2E1) to menthofuran and subsequently to reactive metabolites that bind covalently to liver proteins, causing hepatocellular necrosis.

The dose determines the outcome:

• Weak dilute tea (low pulegone): hepatic metabolism handles the load without damage
• Abortifacient dose of essential oil (high pulegone): overwhelms hepatic capacity, causes liver failure, multiorgan failure, death

The essential oil is not safe for internal use at any dose. This is not relative caution. It is categorical.

The 2,500-year documentation

Pennyroyal’s abortifacient use is documented in Hippocrates (c. 400 BCE), Dioscorides (c. 50 CE), Pliny the Elder (c. 77 CE), and every subsequent era of European herbal writing through to the present. The documentation is explicit: the herb stimulates uterine contractions, and this was used for delayed menstruation and unwanted pregnancy.

Two and a half millennia of consistent documentation about a plant property is unusual. It reflects consistent empirical observation — the mechanism (pulegone causing uterine contractions) is pharmacologically real, not traditional inference.

The modern fatalities occurred when the essential oil became commercially available and was used in the same traditional context at concentrations the traditional dried herb preparations never reached.

What the safe applications are

Insect repellent: Dried herb sachets in wardrobes repel fleas and moths effectively. This is the safest contemporary application. The pulegone is in a concentration that is not a systemic concern at normal household exposure.

Carminative tea (low dose, short course, confirmed not pregnant): ½–1 teaspoon dried herb per cup, steeped 5 minutes. One to two cups daily for digestive gas and bloating. Maximum 1–2 weeks. This application uses concentrations that are not the abortifacient dose.

Diaphoretic (fever, short course): Same dilute preparation combined with elderflower for fever management.

No internal use in pregnancy, ever. No essential oil internal use, ever.

Could you grow this yourself?

Yes — pennyroyal grows readily in damp, moist conditions in full sun to partial shade. It spreads by creeping and forms dense mats. Useful as a lawn substitute in damp areas; walking on it releases the scent. The flowers attract bees.

Keep the dried herb and any preparations away from children and pets; the margin with toxicity is low in small bodies.

Pennyroyal (ペニーロイヤル) in Japan

Available in Japan through Western herbal and aromatherapy channels. Japanese regulatory guidance follows EMA-style caution — internal use of the essential oil is not recommended, and the abortifacient application is not promoted. Pennyroyal essential oil is available in Japanese aromatherapy markets but explicitly labelled for external/diffuser use.

Things you’re probably wondering

Is any amount of pennyroyal safe during pregnancy? No. The European Medicines Agency’s position is that the margin between a therapeutic and a toxic dose is insufficient for any recommendation. In pregnancy, the concern is not hepatotoxicity but uterotonic activity — even modest amounts can stimulate uterine contractions. The abortifacient property that has been documented for 2,500 years is real. Avoid entirely during pregnancy.

Can pennyroyal be used for menstrual irregularity? Contemporary Western herbal medicine does not recommend pennyroyal as a first-line or straightforward treatment given the toxicity profile and the EMA assessment. The traditional emmenagogue application used the same mechanism as the abortifacient application — only dose and timing distinguish them, and this distinction cannot be reliably managed without clinical supervision. Safer alternatives exist for menstrual irregularity (vitex, mugwort, ginger, shepherd’s purse for spotting).

Botanical details

The full compound list

See Also

• Ground Ivy — shares pulegone at lower concentrations; similar but lower-risk carminative applications
• Mugwort — emmenagogue with better safety profile; more appropriate for menstrual applications
• Yarrow — diaphoretic and emmenagogue; lower risk; traditional fever combination

References

• Sullivan JB Jr et al. (1979). Pennyroyal oil poisoning and hepatotoxicity. JAMA, 242(26), 2873–2874.
• Anderson IB et al. (1996). Pennyroyal toxicity: measurement of toxic metabolite levels in two cases and review of the literature. Annals of Internal Medicine, 124(8), 726–734.
• European Medicines Agency. (2011). Public Statement on Mentha pulegium L. EMA/HMPC/337/ 2010.
• Riddle, J.M. (1997). Eve’s Herbs: A History of Contraception and Abortion in the West. Harvard University Press.
• Grieve, M. (1931). A Modern Herbal. Dover.