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Adverse Drug Reactions - Part I

Course Number: 536

Drug-herbal Interactions

Components of grapefruit inhibit cytochrome P450 isoenzyme CYP3A4 and increase the plasma level of a number of drugs; for example, it can significantly enhance the anticoagulant effect of warfarin.13 Ginkgo biloba inhibits platelet function; its use with acetylsalicylic acid (ASA), other nonsteroidal anti-inflammatory agents (NSAIDs), and clopidogrel, which also interfere with platelet aggregation, increases the risk of bleeding.6

St. John’s wort induces the cytochrome P450 isoenzyme CYP3A4 and decreases the efficacy of many drugs, including some prescribed by oral healthcare providers, e.g., benzodiazepines and macrolides antibacterial agents.12,13 Large doses of vitamin C acidify the urine, inhibit the excretion of weak acids such as NSAIDs, acetaminophen, and tetracyclines (e.g., doxycycline and minocycline) and thus increase the plasma levels of these and other drugs.