The stems are square in cross-section and can vary from hairless on some plants to densely hairy on others, with a green to sometimes red or purplish colour.

The small (6 mm) flowers are densely packed in whorls at the nodes, widely separated, above pairs of leaf-like bracts.

The calyx is a ribbed tube about 3 mm long, with five triangular teeth, the lower two being narrower and slightly longer than the upper three; it is hairy both on the inside and the outside.

[citation needed] The habitat of pennyroyal is seasonally damp pasture, where standing water over the winter leaves bare ground in the summer, and where livestock preferentially graze other plants.

An analogue of this habitat is found on roadsides, where trampling or disturbance of the ground produces similarly bare soil, particularly where there is waterlogging in winter.

[11] In California, where it is considered an invasive species, it occupies a similar niche, in seeps, streamsides, vernal pools and swales, marshes, and ditches.

Pliny the Elder, in his encyclopedia Naturalis Historia (Natural History), described the plant as an emmenagogue, and that it also expelled a dead fetus.

Roman and Greek writers Quintus Serenus Sammonicus and Aspasia the Physician however both agreed that pennyroyal, when served in tepid water, was an effective abortive method.

[20] A medical text on gynecology attributed to Cleopatra (though it was actually written by a female Greek physician Metrodora) recommends the use of pennyroyal with wine to induce abortions.

[22] In addition to its abortive properties, Culpeper recommends its use for gastrointestinal ailments, such as constipation and hemorrhoids, as well as itching and blemishes to the skin, and even toothaches.

[26] The fresh or dried leaves of pennyroyal have also been used when treating influenza, abdominal cramps, to induce sweating, as well as in the treatment of diseases such as smallpox and tuberculosis.

[28] In Italy the fresh leaves of this plant, which in Rome and the surrounding area is called menta romana, are used in the capital's cuisine to flavour lamb and tripe.

Women who struggle with regulating their menstrual cycle or suffer from a cystic ovary syndrome may choose to drink pennyroyal tea.

[32] Rennie's 1833 supplement to the pharmacopeias admitted its use as an "expectorant, diuretic, and emmenagogue" in doses of 10 grains to 1 scruple (0.6–1.3g) of the powdered dried herb, but dismissed the use of the Pennyroyal Water (Aqua Pulegii) as "popularly but erroneously supposed" to be an abortifacient [no mention is made of toxicity].

[35] Symptoms that may persist after ingesting a small dose (<10 mL) of pennyroyal oil are nausea, vomiting, abdominal pain and dizziness.

The exocyclic double bond found in pulegone is vital to the activation and binding mechanism of the molecule and causes it to be an effective hepatotoxin.

Studies conducted on rats show that one of the main effects is the inhibition of contractile activity in the myometrium and death by kidney failure.

[37] Both in vitro and in vivo studies have found the pulegone metabolite menthofuran to be an inhibitor of CYP2A6, accounting for a significant degree of pennyroyal's hepatotoxicity.

[39] Additionally, pulegone and menthofuran may deplete glutathione levels, leaving hepatocytes vulnerable to free radical damage.

[30] Case studies involving pennyroyal poisonings have reported the use of gastric lavages and administration of emetics or vomiting inducing agents, or absorbents like activated charcoal.

[6] As glutathione depletion has been shown to regulate liver toxification, administration of N-acetylcysteine in similar doses as given for acetaminophen toxicity have been given to patients.