If you are currently being treated with any of the following medications, you should not use garlic supplements without first talking to your healthcare 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, 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 should refrain from consuming large quantities of garlic, either fresh or commercially prepared.

Diabetes medications
When used with a class of medications for diabetes called sulfonylureas, garlic may lower blood sugar considerably. Medications from this class include chlorpropamide, glimepiride, and glyburide. When using garlic with these medications, blood sugars must be followed closely.

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

Other
It is thought that garlic may behave similarly to a class of cholesterol lowering medications called statins (such as atorvastatin, pravastatin, and lovastatin) and to a class of blood pressure lowering medications called ACE inhibitors (including enalapril, captopril, and lisinopril). It is not known, therefore, whether it is safe to take this supplement in large quantities with these medications or not. This possible interaction has never been tested in scientific studies.