Asteraceae

Silybum marianum

Milk thistle · St Mary's thistle

Mixed Low

Part used: seed
Native range: Mediterranean and southwest Europe; a serious naturalised weed across southern Australia
Morphology: A robust annual or biennial thistle to two metres, with glossy spiny leaves marbled with white veins and solitary purple flower heads.
Also known as: Carduus marianus

What is in it

flavonolignans silybin, silychristin, silydianin

flavonoids

fatty oil

Evidence

Persistently popular and persistently inconclusive. Cochrane reviews of silymarin in alcoholic and viral liver disease find no convincing effect on mortality or histology. Intravenous silibinin has a genuine and well-established use as an antidote in Amanita phalloides poisoning, which is a different preparation and a different situation.

Safety

Well tolerated; mild laxative effect is the commonest complaint. Allergy in people sensitive to other Asteraceae. Oral bioavailability of silymarin is poor, which is a plausible part of why oral trials disappoint.

Interactions: Weak inhibition of some cytochrome enzymes; clinically significant interactions are not well established

Regulatory: Permitted as a listed medicine ingredient in Australia. A declared weed in several Australian states.

How the traditions classify it

Galenic Classed as hot and dry, and used for liver and spleen complaints. The white leaf veins were read as the Virgin’s milk, which is a doctrine-of-signatures reading and the source of both common names.

Three systems, rating the same plant. Where they disagree, none of them is correcting the others — they are separate frameworks that arrived at their categories independently.

Historical use

Recorded by Dioscorides and Pliny, and consistently associated with the liver in European practice from the classical period. Also eaten as a vegetable.

Recorded as history and attributed where possible. Long use establishes that a plant was widely used, not that it worked, and not that it was safe.

Worth knowing

Milk thistle earns its place partly as a doctrine-of-signatures story that happens to have pointed somewhere real.

The white marbling on the leaves was read as milk spilled by the Virgin, and milk meant nourishment, and nourishment meant the liver. That is not reasoning that should work. And the plant does contain compounds with genuine hepatoprotective activity — silibinin given intravenously is a standard treatment for death cap poisoning.

What has not survived is the oral preparation for chronic liver disease, which is what almost everyone actually buys it for. The compound is real, absorbed badly, and the trials are flat.

Nomenclature verified against GBIF backbone taxonomy.