The milk thistle plant commonly grows from 2 to 7 feet in height, with spiny leaves and reddish-purple, thistle-shaped flowers. It has also been called wild artichoke, holy thistle, and Mary thistle. Native to Europe, milk thistle has a long history of use as both a food and a medicine. At the turn of the twentieth century, English gardeners grew milk thistle to use its leaves like lettuce (after cutting off the spines), the stalks like asparagus, the roasted seeds like coffee, and the roots (soaked overnight) like oyster plant. The seeds and leaves of milk thistle were used for medicinal purposes as well, such as treating jaundice and increasing breast milk production.

German researchers in the 1960s were sufficiently impressed with the history and clinical effectiveness of milk thistle to begin examining it for active constituents. In 1986, Germany's Commission E approved an oral extract of milk thistle as a treatment for liver disease. However, the evidence that it really works remains incomplete and inconsistent.

Based on the extensive folk use of milk thistle in cases of jaundice, European medical researchers began to investigate its medicinal effects. It is currently used to treat alcoholic hepatitis, liver cirrhosis, liver poisoning, and viral hepatitis, as well as to protect the liver in general from the effects of liver-toxic medications. However, despite this wide usage, there is no definitive evidence that it is effective.

Standardized milk thistle extract is known as silymarin. Silymarin itself is a mixture of at least seven chemicals. The most active of these chemicals is commonly known as silibinin. But, silibinin too is, in fact, a mixture, comprising the two related substances silibinin A and silibinin B.48 When injected intravenously, silibinin is thought to act as an antidote to poisoning by the deathcap mushroom, Amanita phalloides. Animal studies suggest that milk thistle extracts can also protect against many other poisonous substances, from toluene to the drug acetaminophen.2-7One animal study suggests that milk thistle can also protect against fetal damage caused by alcohol.8

Silibinin is hypothesized to function by displacing toxins trying to bind to the liver as well as by causing the liver to regenerate more quickly.9 It may also act as an antioxidantand also stabilize liver cell membranes.10,11

In Europe, milk thistle is often added as extra protection when patients are given medications known to cause liver problems. However, milk thistle failed to prove effective for preventing liver inflammation caused by the Alzheimer's drug Cognex (tacrine).12

Milk thistle is also used in a vague condition known as minor hepatic insufficiency, or "sluggish liver."13 This term is mostly used by European physicians and American naturopathic practitioners—conventional physicians in the US don't recognize it. Symptoms are supposed to include aching under the ribs, fatigue, unhealthy skin appearance, general malaise, constipation, premenstrual syndrome, chemical sensitivities, and allergies.

Milk thistle may also offer some protection to the kidney.14

Highly preliminary evidence hints that milk thistle might help reduce breast cancerrisk.15 Milk thistle is sometimes recommended for gallstones and psoriasis, but there is little to no evidence as yet that it really helps these conditions.

In one small, placebo-controlled trial, the topical application of milk thistle with methylsulfonylmethane (MSM) for 1 month appeared to be effective in the treatment of 46 subjects with the skin condition rosacea.50

A small preliminary study investigated whether milk thistle can help to relieve obsessive compulsive disorder(OCD).52 Thirty-five adults with OCD were randomized to receive milk thistle (600 mg/day) or the medication fluoxetine (30 mg/day), which is commonly used to treat OCD. At the end of the 8-week trial, researchers did not find any significant differences between the two groups.

Two, small randomized, placebo-controlled trials (with a total of 89 people) found that milk thistle helped to improve blood sugar control in people with type 2 diabetes.55