Drug Interactions

A drug interaction is a change in the way a drug acts in the body when taken with certain other drugs, nutritional supplements, herbals, or foods. Drug interactions may cause the drug to be more or less effective, or cause unexpected effects on the body. Some medicines can also interfere with laboratory test results.

The risk of drug interactions increases greatly with the number of medicines that a person takes. Some drug interactions are relatively minor and are unlikely to cause harm. In some rare cases, however, drug interactions can be fatal.


Examples of Drug Interactions

Pam has been taking Lanoxin® (digoxin) for the past two years to help manage her atrial fibrillation (a heart rhythm problem). She recently developed a respiratory infection and her doctor prescribed erythromycin (an antibiotic). Two days later, she began feeling tired and saw halos around lights. When she called her doctor, he realized that the erythromycin had caused her digoxin blood levels to go up, causing toxicity from the digoxin. He stopped the erythromycin and changed her to a different antibiotic.

Ralph has been taking Zocor® (simvastatin) for several years to help keep his cholesterol level down. He recently retired and moved to Florida, where he began to enjoy eating fresh grapefruit on a regular basis. Soon afterwards, he began having painful muscle aches in his back and legs. He mentioned this to the pharmacist when he went to get his Zocor refilled and she told him that grapefruit and grapefruit juice can interact with his cholesterol medicine. The result of this interaction was that the blood level of his cholesterol medicine went up, causing side effects.

Nancy has been taking Coumadin® (warfarin) since she developed a blood clot in her leg. Nancy has been worried lately about whether her memory might be slipping and a neighbor told her that ginkgo biloba, a herbal product that can be purchased over-the-counter, might help improve her memory. A few days after starting the ginkgo biloba, her gums began bleeding while she was brushing her teeth. She wondered if this bleeding might be related to starting the ginkgo. When she looked it up on the internet, she found that ginkgo biloba can indeed increase the risk of bleeding in persons who take Coumadin®.


Steps You Can Take to Decrease Risk of Drug Interactions

One reason that older adults may be at greater risk for drug interactions is because of the number of physicians that are seen for various medical problems. One doctor may treat the heart problem, another doctor may treat the arthritis, etc. Sometimes, doctors may prescribe a medicine for you without knowing what medicines the other doctors are prescribing.

One way you can help prevent this problem is keep a list of all the medicines you are taking. Here is a link to a medication list form (PDF) that you can use to keep track of your medicines.

Another way to reduce your risk of harm from drug interactions is to get all your prescriptions filled at one pharmacy. When a combination of pharmacies is used to obtain medicines (such as mail order plus a local pharmacy), no pharmacy will have a complete record of all the medicines you take. This can increase the risk that you will accidentally take medicines that interact.

Ask your pharmacy about how they check for drug interactions. Most pharmacies have computer software that helps alert them to drug interactions. This software should be kept up to date and should be programmed to provide warnings about any dangerous drug interactions.

Older persons, especially those who take multiple medicines, should have their medicines reviewed by a physician or pharmacist at least once per year. Put all the medicines you take, including over-the-counter and herbals or vitamins, into a brown paper bag and take them to your doctor or pharmacist for a thorough review. Be sure to make arrangements ahead of time, since a thorough review of medicines may take at least 30 to 60 minutes in some cases.

Learn about the value of senior care pharmacists and find one in your area.


Resources on Drug Interactions

You can enter a list of medicines and check for drug interactions at this Web site:

You also can obtain information on interactions among prescription drugs, over-the-counter drugs, and herbals and supplements at this AARP Web page:

Check this Web page from the Mayo Clinic to learn more about grapefruit juice interactions:

Read more about drug interactions and side effects involving Coumadin® (warfarin) at this Mayo Clinic Web page:


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