Peppermint

Peppermint (Mentha × piperita) is a perennial herb, a hybrid between water mint (Mentha aquatica) and spearmint (Mentha spicata), growing 30–90 cm and sometimes up to 1 m tall. The stems are square, erect and branched, often tinged red-purple; the opposite leaves are ovate to lanceolate, 4–9 cm long and 1.5–4 cm wide, with serrated margins, dark green surfaces and scattered oil glands (peltate trichomes). Small lilac flowers are grouped into terminal spikes in mid- to late summer; because it is a hybrid, the flowers are largely sterile and viable seed is rarely produced. The plant spreads widely by creeping stolons that form dense mats, so it is propagated mainly by cuttings and division.

According to Kew's Plants of the World Online the native range of Mentha × piperita is Europe to Central Asia, in the temperate biome, with the hybrid formula M. aquatica × M. spicata. It is a long-cultivated plant that has become established and naturalised in many humid temperate regions worldwide. Today the main producing areas are the United States (Washington, Oregon and Idaho), India (the plains of Uttar Pradesh), Europe (the UK, France and Germany), China and Egypt; it is also grown in Vietnam, mainly on a small scale for fresh leaves and oil distillation.

Steam distillation of the leaves yields peppermint oil whose main components are menthol (30–55%) and menthone (14–32%), accompanied by menthyl acetate, menthofuran (1–9%), 1,8-cineole, limonene, isomenthone, neomenthol and small amounts of pulegone; menthofuran is a characteristic marker of peppermint and rises when the crop is harvested too early. (−)-Menthol predominates; it activates the TRPM8 cold receptor (McKemy et al., Nature, 2002) to produce the cooling sensation and inhibits L-type calcium channels in smooth muscle, giving the oil its antispasmodic action. ISO 856 specifies the compositional ranges for the two traditional trade types, American and English (Mitcham) peppermint oil.

Peppermint has relatively strong clinical evidence for digestive complaints: the meta-analysis by Alammar et al. (BMC Complementary and Alternative Medicine, 2019), covering 12 randomised trials, showed that enteric-coated peppermint oil is safe and effective at relieving abdominal pain and global symptoms in adults with irritable bowel syndrome (IBS), and the systematic review and meta-analysis by Khanna et al. (Journal of Clinical Gastroenterology, 2014) reached a similar conclusion for short-term treatment. The mechanism is thought to involve menthol blocking calcium channels and relaxing intestinal smooth muscle. In addition, the review by McKay and Blumberg (Phytotherapy Research, 2006) covers the antispasmodic, antibacterial and antioxidant activities of peppermint tea, while the oil is used widely as a flavouring (confectionery, chewing gum, toothpaste, mouthwash), in cough remedies and cooling balms, and diluted topically for tension headache.

On safety, peppermint is GRAS as a flavouring and has been assessed by the Cosmetic Ingredient Review (International Journal of Toxicology, 2001). Peppermint oil can, however, cause heartburn and gastro-oesophageal reflux, so people with severe reflux should avoid it; enteric-coated capsules are designed to prevent release in the stomach. Caution is needed with gallstones or bile-duct obstruction, and the oil should not be applied to the face or nose of infants because menthol can trigger reflex apnoea. Pulegone is hepatotoxic at high doses in animals, but its level in peppermint oil is low (unlike pennyroyal); prolonged high-dose use is still best avoided. For conservation, the IUCN lists the hybrid as Not Evaluated and it is not in CITES; as a sterile, vegetatively propagated cultigen grown worldwide, peppermint is not threatened in the wild and its genetic resources are maintained mainly through clonal lines.