Garlic may alter the function of certain prescription medications. If you are being treated with any of the following medications, you should not use garlic supplements without first talking to your health care provider.

Antiplatelet medications -- Garlic may exaggerate the activity of medications that inhibit the action of platelets in the body. Examples of such medications include indomethacin, dipyridamole, Plavix, and aspirin.

Blood-thinning medications -- There have been reports of a possible interaction between garlic and warfarin that could increase the risk of bleeding in people taking this blood thinning medication. Therefore, when taking medications that may thin the blood, such as aspirin and warfarin, you not use garlic supplements unless you are under the supervision of a doctor.

Protease inhibitors -- Garlic may reduce blood levels of protease inhibitors, a medication used to treat people with the human immunodeficiency virus (HIV). Protease inhibitors include indinavir, ritinavir, and saquinavir.