Ask the Doctors

Dear Doctor: What are the pros and cons of taking milk thistle?

Dear Reader: Let's begin with some background on this purple-flowered plant that most people in this country think of as an invasive weed. It's a member of the thistle family, so-named for the sharp projections upon the leaves and stems. Although it originated in Europe, milk thistle (Silybum marianum) has made its way to many parts of the world, including the hills of Southern California, where I've pricked my legs on it many times while hiking. Nuisance or not, however, the seeds of the plant have been used for medicinal purposes for more than 1,000 years. That's because they contain silymarin, a mixture of plant chemicals known as flavonolignans.

Milk thistle is often touted as a supplement to protect the liver. Before today's more effective treatments for hepatitis C, many patients used milk thistle in an attempt to limit liver damage from the virus. Although silymarin has been shown to have antiviral and antioxidant activity, and theoretically could limit tissue and cellular damage caused by the virus, the majority of studies of silymarin and hepatitis C have not been well-controlled.

Most notably, a 2014 review combined five randomized controlled trials of milk thistle extracts; three of the studies -- involving 216 patients -- compared changes in hepatitis C viral loads among people taking either silymarin or a placebo. The review found a slight decrease in the viral loads of people using silymarin, but it was not of statistical significance. Nor did the compiled studies find any changes in the elevation of liver enzymes, a marker of liver inflammation, in people taking silymarin.

Of course, other conditions affect the liver as well, such as alcohol liver disease and fatty liver. Research has shown some improvement with silymarin use in liver markers in both of these conditions, but the studies have been limited.

Note, however, that one of the flavonolignans, silybin, may have particular potential in limiting damage to the liver. In one study in mice, the addition of silybin prior to the exposure of toxic mushrooms decreased the chance of severe liver damage. Also, case reports have shown that the administration of silybin extract increased the survival rate among people with death cap mushroom poisoning. Further, the larger compound, silymarin itself, can decrease liver toxicity in mice given high doses of acetaminophen.

The larger compound has also been studied in people with diabetes. The thinking is that its anti-oxidant and anti-inflammatory effects could theoretically reduce complications of the disease. And, in fact, preliminary studies in people with diabetes have shown that it does have beneficial effects upon the kidneys, nerves and the retinas.

Silymarin may even have anti-cancer properties, but this needs to be studied further in humans.

As for the side effects of silymarin, they include abdominal discomfort, headache, muscle aches and fatigue. Because the compound acts within the liver, it could theoretically interact with multiple medications metabolized there. Thus far, however, studies of people taking up to 140 milligrams per day of silymarin have not shown significant drug interactions.

In summary, the components of milk thistle supplements do seem to have some beneficial effects. Future studies will need to assess its full benefits, or risks, in those with liver disease, diabetes and cancer.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)

More like Ask the Doctors