Pennyroyal (Hedeoma pulegioides), known as American pennyroyal, and Mentha pulegium, known as English or European pennyroyal, are both members of the Lamiaceae or mint family. These two beneficial herbs, though classified in different genera, have similar chemical constituents and medicinal properties.
American pennyroyal is also known as mock pennyroyal, mosquito plant, fleabane, tickweed, stinking balm, and hedeoma. This aromatic American native thrives in limestone-rich soil, in fields, and in sunny patches of open woodlands throughout North America.
American pennyroyal was used extensively by Native Americans to treat a variety of ailments from headache and stomach distress to itching, watery eyes, and fevers.
For external use, the leaves were crushed and applied to the skin to repel mosquitoes and other insects. American pennyroyal came to be called squawmint and squaw balm because of its traditional use by native women to promote menstrual flow.
Women in some Native American tribes reportedly drank hot pennyroyal tea regularly as a method of contraception. Pennyroyal was listed as a medicinal drug in official publications from 1831–1931. It was included in the U.S. Pharmacopoeia as an abortifacient (an agent that induces abortion) until 1931.
American pennyroyal is an annual mint with small, oval leaves arranged opposite each other on a square stem. Leaves are entire and may be sparsely-toothed or smooth on the margins. The erect stems grow to 1 ft (31 cm) high from a many-branched root system.
The tiny blue-violet flowers grow in whorls from the leaf axils on the top half of the stems. The fragrant herb blossoms in midsummer. The entire plant exudes a strong, acrid aroma and has a mint flavor. The scent is offensive to fleas, chiggers, mosquitoes, and other irritating insects.
European pennyroyal, also known as English pennyroyal, is a perennial mint native to Europe and Asia. The herb has naturalized throughout North America since its introduction to the continent by early European colonists.
European pennyroyal was mentioned in Greek literature as early as 421 B.C. in the plays of Aristophanes where it was noted for its use as an abortifacient. In the first century A.D. the herbalist and physician Pliny wrote of pennyroyal’s action to repel fleas. The specific name for the herb is from the Latin word pulex, meaning flea.
European pennyroyal thrives in moist areas along stream banks, around ponds, in irrigated fields, and in boggy grasslands. This growing habit is reflected in some of European pennyroyal’s other common names, including run-by-the-ground, lurk-in-the-ditch, and pudding grass.
European pennyroyal can be distinguished from the American native pennyroyal not only by its preferred habitat, but also, with careful observation, by its appearance. European or English pennyroyal hugs the ground where it grows, with only the flower stalk rising to a height of about 1 ft (31 cm).
The oval leaves are opposite along the square stem, but are smaller than those of the American pennyroyal, measuring about 0.5 in (1.3 cm) long. The tiny, tubular blossoms each have four stamens in the European herb, and bear only two stamens in the American native.
Pennyroyal has been used traditionally as a stimulating tea to relieve digestive disorders, gall bladder disorders, gout, nausea, and nervous conditions. Pennyroyal leaf, prepared as a hot infusion, will promote perspiration.
Some herbalists suggest the additional treatment of a hot footbath while drinking the herbal infusion as a remedy at the onset of colds and flu. Pennyroyal may relieve headache, bring down fever, and quiet coughs.
It has also been used to treat bronchitis and sinusitis. As a carminative (gas-reliever), pennyroyal is considered an effective remedy for flatulence, a virtue it shares with other mints. The herbal infusion has also been used traditionally to treat suppressed menstruation.
By far the most controversial and dangerous use of pennyroyal is as an abortifacient. Its emmenagogic properties stimulate uterine contraction and promote menstrual flow. The essential oil has been used for centuries to induce abortion.
This use of the essential oil of pennyroyal is extremely risky, and has sometimes been lethal to both the mother and the fetus. The U.S. Food and Drug Administration reported on a 1998 fatal case of pennyroyal overdose in a self-induced abortion. Both pennyroyals contain as much as 85% of the toxic phytochemical pulegone in the essential oil.
Pennyroyal is also considered potentially dangerous because of its hepatotoxicity, or ability to harm the liver. Of four cases of pennyroyal poisoning reported in San Francisco in 1996, one patient died from liver damage. As of late 2001, researchers are studying the pathways of pulegone metabolism in the human body in order to determine the degree of toxicity more precisely.
The best use for this potent herb is an external application as an insect repellent to deter mosquitoes, fleas, chiggers, and other pests. It is also soothing as a skin wash to relieve itching and rash. Pet collars, woven from the freshly gathered stems and leaves, will deter fleas, and bunches of the herb, hanging to dry, will also keep pests at bay.
Many commercial products contain the oil of pennyroyal in insect-repellent preparations. Other chemical constituents in pennyroyal include tannins, such as rosmaric acid, and flavonoids, including diosmin and hesperidin.
The essential oil of pennyroyal and the fresh or dried leaves and stems are medicinally active. Gather fresh leaves in the summer, on a dry and sunny day when the herb is in blossom. Hang bundles of the herb to dry in a light, airy room out of direct sunlight. When the herb is thoroughly dry, strip the leaves from the stems and store in tightly sealed, clearly labeled, dark-glass containers.
Infusion: Place 2 oz of fresh, or 1 oz of dried, pennyroyal leaves in a warmed glass container. Bring 2.5 cups of fresh, nonchlorinated water to a boil and add it to the herbs. Do not boil the tea. Cover and infuse the tea for about 10 minutes. Strain.
The prepared tea will store for about two days in the refrigerator. This infusion may be used externally as a soothing skin wash. According to some herbalists, pennyroyal leaf infusion may also be safely consumed as a medicinal tea, taking up to two cups throughout the day.
Others, however, including the PDR For Herbal Medicines, recommend that pennyroyal not be ingested due to its hepatotoxicity. Other non-toxic herbs, such as peppermint (Mentha piperita) and spearmint (Mentha spicata) may be used to remedy many of the same conditions that pennyroyal treats without the toxic risk.
Pregnant women should never ingest pennyroyal, particularly the oil, nor should they apply the oil externally as it may be absorbed through the skin. Pennyroyal essential oil contains as much as 85% of the ketone pulegone, an extremely toxic phytochemical.
Overdose of the essential oil has been reported to cause severe liver damage, coma, and death. Quantities as small as 0.5 tsp of the essential oil have caused extremely toxic reactions. The effective abortifacient dosage is dangerously close to the lethal dose. Women have died when attempting to induce abortion by ingesting pennyroyal oil.
American pennyroyal contains twice as much of the toxic volatile oil as European pennyroyal. The PDR For Herbal Medicines recommends that the drug not be used because of its hepatotoxicity, although with proper dosage and administration of the foliage drug, poisoning is not likely.
Contact dermatitis is possible when using crushed leaf or the undiluted oil extract on the skin to repel insects.