Fish mint

Fish mint is a perennial herb 20–60 cm tall with stems that creep then stand erect and white rhizomes underground (a rhizomatous geophyte per POWO). Leaves are alternate and short-stalked, heart-shaped with a pointed tip, 3–8 cm long, strongly veined and often reddish-purple beneath. The tiny flowers form a dense terminal spike, subtended by four white petal-like bracts that look like four petals around the small yellowish-green flower spike; there are no true petals. The fruit is a small capsule with many tiny seeds.

According to Kew's Plants of the World Online, the native range of fish mint runs from the Himalaya to temperate East Asia and Indo-China — covering Assam, the East and West Himalaya, Nepal, Bhutan, Bangladesh, Myanmar, Thailand, Cambodia, Vietnam, China (north-central, south-central, south-east, Hainan, Tibet), Taiwan, Japan, Korea and the Ogasawara Islands. It grows along stream edges, in wet woodlands, damp grassy places, paddy-field margins and roadsides, from near sea level up to 2,500 m. Fish mint has been introduced and naturalised in Austria, Germany, Great Britain, Ireland, Italy, Czechia–Slovakia, Costa Rica, Minnesota and New York (USA), as well as in Australia, New Zealand and several Pacific islands, where it is often regarded as a fast-spreading weed. In Vietnam it is grown and found wild in damp, shaded spots around homes and along ditches.

Its chemistry spans essential oil, flavonoids, phenolic acids, polysaccharides and alkaloids. The volatile oil makes up about 0.05% by weight, most abundant in the flowers and then the leaves, stems and rhizomes; its main constituent is 2-undecanone (methyl nonyl ketone, 19–56% in the aerial parts and 29–42% in the rhizomes), together with decanoyl acetaldehyde (houttuynin, 3-oxododecanal) — the compound behind the fishy smell and an antibacterial agent that is highly unstable and readily converts to 2-undecanone on heating. Others include myrcene, α-pinene, β-pinene, limonene, decanal, camphene, bornyl acetate and caryophyllene; one review catalogued as many as 346 volatile components. The main flavonoids are quercetin, quercitrin, isoquercitrin, rutin, hyperin, afzelin and kaempferol, while the phenolic acids include chlorogenic, neochlorogenic, caffeic, protocatechuic and gallic acid. The plant also holds phenanthrene alkaloids (aristolactam A, aristolactam B, piperolactam A, 3-nonylpyrazole) and polysaccharides — groups tied to its antibacterial, anti-inflammatory and immunomodulatory effects.

Fish mint is a long-standing food-and-medicine plant in Asia; the Chinese Pharmacopoeia (2020 edition) records it as clearing heat and toxins, reducing abscesses and draining pus, and promoting diuresis, used for phlegm-heat cough, dysentery and boils. Recent reviews (Pharmaceuticals, 2022; Journal of Ethnopharmacology, 2023) compile the antibacterial, antiviral, anti-inflammatory, antioxidant, anticancer and immunomodulatory activities of its extracts and isolated compounds: sodium houttuyfonate with EDTA-Na2 inhibited MRSA (raising mouse survival to 75% versus 43.75–50% for either agent alone), and aqueous extracts inhibited multidrug-resistant E. coli; the flavonoids houttuynoid A–E inhibited HSV-1 with IC50 values of about 23.5–59.9 µM (Chen et al., 2012), and houttuynoid B showed anti-Zika activity (2019). A purified microemulsion-encapsulated essential oil showed anti-Coxsackie antiviral activity with an IC50 of 0.35 µg/mL and a selectivity index of at least 519 (2017). In everyday use the young leaves are eaten raw, in salads, wraps and soups, while the whole herb is decocted or made into extracts, capsules, mouthwash, fermented drinks and cosmetic ingredients.

On safety, fish mint taken as food or a decoction is generally considered safe, but the INJECTABLE form caused serious harm: between 1988 and April 2006 China's adverse-reaction monitoring centre recorded more than 5,000 adverse reactions (222 serious, including deaths from anaphylactic shock), and on 1 June 2006 the State Food and Drug Administration suspended seven injectable fish-mint preparations, later permitting only intramuscular use with an allergy warning. The cause has been linked to impurities and degradation products of unstable houttuynin plus the excipient polysorbate-80; therefore injectable preparations should NOT be used, and caution is advised for allergy-prone individuals and pregnant women. On conservation, POWO records the species as widespread and common, not evaluated against IUCN Red List criteria, and not at global risk; conservation mainly means keeping local forms and controlling its spread outside the native range.