Herbs, vitamins, and other dietary supplements may augment or antagonize the actions of medications prescribed by your doctor. What research has shown is that in adults, St. John's wort is the supplement that has the most documented interactions with drugs. There could be serious consequences when the supplements interact with medications that prevent clotting, medications for heart conditions, medications for diabetes and medications for conditions related to HIV.
Several online resources allow you to check for interactions between drugs and supplements you are taking. Most analyze drug-drug interactions. Information on drug-supplement, drug-food, or supplement-supplement interactions is also available on several sites. The websites include the following:
Drugs.com
Medscape.com
Memorial Sloan Kettering Cancer Center
Rxisk.org
Rxlist.com
Webmd.com