This fairly typical thistle has red to purple flowers and shiny pale green leaves with white veins.

Milk thistle is an upright herb that can grow to be 30 to 200 cm (12 to 79 in) tall and has an overall conical shape.

The leaves are oblong to lanceolate and 15–60 cm (6–23+1⁄2 in) long and typically pinnately lobed, with spiny edges like most thistles.

[4] The flower head is surround by bracts which are hairless, with triangular, spine-edged appendages, tipped with a stout yellow spine.

The fruits are black achenes with a simple long white pappus, surrounded by a yellow basal ring.

[6] Silybum marianum is native from around the Mediterranean and much of Europe to Central Asia and India; in Africa it reaches as far south as Ethiopia.

S. marianum has been widely introduced outside its natural range, for example into North America, Hawaii, Australia, New Zealand, and Colombia where it is considered an invasive weed.

[10] Silybum marianum establishes itself in sunny, warm ruderal meadows in regularly disturbed places such as rubble deposits, at the foot of south-exposed walls or villages and on urban fallow land or on cattle pastures.

[23] Eyelid edema, ocular pruritus, dry eye, diplopia, and blurred vision are among the reported complications based on registered side effects in the WHO global database of adverse drug reactions.

[4] When potassium nitrate is eaten by ruminants, the bacteria in the animal's stomach breaks the chemical down, producing nitrite ions.

[25] Although potentially allergenic, the leaves and stems can be gathered ahead of bloom, the spines removed, and boiled with salt.