Taking Medicines

Frequently Asked Questions

16. What are drug interactions?

Drug interactions occur when a medicine interacts chemically with another drug or with certain foods. These interactions change the way your body handles one or both medicines, which can change the safety or effectiveness of either or both drugs.

In some cases, the overall effect of an interaction is greater than desired. For example,

  • combining aspirin with blood-thinning drugs such as Coumadin┬«, also called warfarin, can cause serious bleeding
  • mixing Viagra┬«, also called sildenafil, and the heart drug nitroglycerin can cause blood pressure to plunge to dangerously low levels.

Some foods can also prevent other drugs from working properly. For example,

  • calcium-rich dairy products or certain antacids can prevent antibiotics from being properly absorbed into the bloodstream
  • a single glass of grapefruit juice can raise the level of some medications in the blood, particularly drugs commonly used to treat heart conditions. (Learn more about the "grapefruit juice effect.")

Even if a product is not called a drug, your body handles it the same way it handles drugs. Some herbal and other substances can interact in potentially dangerous ways with prescription drugs or other over-the-counter products. For example,

  • ginkgo biloba can reduce the effectiveness of blood-thinning medications and raise the risk for serious complications such as stroke.

Always inform your doctor or pharmacist about all medicines you are taking. Ask whether the medications might interact with any foods or other over-the-counter drugs or supplements you may also be taking.