Milk thistle is available in health food stores and in drugstores that sell herbal products. Extracts used in clinical trials are standardized to 80 per cent of an active ingredient called silymarin. Milk thistle occasionally has been reported to cause mild diarrhea but has no other reported side effects. It may, however, cause a reaction in people who are allergic to plants from the Asteraceae (daisy) family.
People taking medications, including antiretroviral drugs, should be cautious about the use of milk thistle as it may interact with them. This is because herb-drug interactions can lead to increased side effects or reduced effectiveness of drug therapy (see the section in this guide called Herb-Drug Interactions).