Most people appear to tolerate garlic well, but some individuals may experience gastrointestinal irritation. People who handle garlic products occasionally develop a skin reaction on exposure. Ingestion of fresh garlic and most extracts may also result in a characteristic breath odor. Although there are no studies of drug interactions, in theory garlic may increase the risk of bleeding in people taking aspirin or blood-thinning medications such as coumadin. There is also a possibility that garlic could interact with drugs such as DiaBeta or Glucotrol that lower blood sugar.
Careful monitoring is recommended for anyone combining garlic products with prescription drugs.
Garlic appears to inhibit an enzyme called CYP 2E1. In most cases, this interference is viewed positively, because CYP 2E1 can make carcinogens more dangerous. However, CYP 2E1 is also involved in the metabolism of acetaminophen and a muscle relaxant called chlorzoxazone. These drugs could possibly remain in the body longer in people who are taking or eating garlic. People with low thyroid function should also be aware that concentrated garlic products may keep the thyroid gland from utilizing iodine properly.