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Drug Interactions

Drug Interactions


Drug interactions occur when one drug in the body affects another drug that a person is taking. The interaction can take many forms, and may be helpful or harmful. Drugs that are known to interact are sometimes given together in order to have a positive effect.

What is the information for this topic?

Other drug interactions, however, can cause serious problems. The more drugs a person takes, the more likely a drug interaction is. Drug interactions can occur with many types of substances. These include prescription medications, over-the-counter medications, herbal remedies, supplements and vitamins. Homeopathic remedies, alcohol, tobacco, and illegal drugs can also interact with each other and with prescription medications.

How do drug interactions occur? There are several ways in which drug interactions can take place. Drugs can affect the levels of other drugs in the body. For example, drug A might interfere with the way drug B is absorbed into the bloodstream through the gut. Or drug A might block the metabolism, or the breakdown, of drug B by the liver. Drug A might block drug B from being eliminated from the urine.

Drugs can also affect the ability of another drug to work at the cell level. For example, one drug may push the other drug off the cell and make it less effective. Two drugs that work in a similar way can cause greater effects when given together than either of the drugs alone. There are a number of other kinds of interactions.
Drug interactions can happen in the following ways:
  • Drug A increases the effect of drug B. This can make drug B more likely to cause side effects or toxic effects. For example, ketoconazole, a medication used to treat fungal infections, can increase the levels of a blood-thinning medication called warfarin, potentially causing serious bleeding.
  • Drug A decreases the effect of drug B. This may make drug B less effective or even ineffective. For example, an antibiotic called rifampin can cause birth control pills to be broken down (metabolized) too quickly. A woman taking both of these medications risks becoming pregnant. Another example is butorphanol (i.e. Stadol), which can push morphine off nerve cells and reduce its ability to block pain.
  • Drug A and drug B have additive effects, which can be helpful or harmful. For example, both aspirin and warfarin thin the blood. In some situations, taking both medications may be beneficial. But in other cases, it may lead to an increased risk of serious bleeding.
  • Drug A and drug B have opposite effects. This may cause an unpredictable response. For example, people on medication to control high blood pressure must be careful if they take ephedrine for nasal congestion because it can raise blood pressure.

What can patients do about drug interactions? When a new medication is prescribed, a person should tell his or her healthcare professional what other drugs he or she is taking. To avoid serious drug interactions, one must mention medications that have been prescribed by other healthcare providers. A person cannot afford to conceal information about any substances he or she may take, including birth control pills, over-the-counter medications, herbal remedies, complementary therapies, or illegal drugs.

The American Society of Anesthesiologists recommends discontinuing all herbal supplements at least two weeks before planned surgery due to potential drug interactions. An individual who takes over-the-counter medications should read the warning labels on the package. Sometimes, important drug interactions are listed on the box. Those who take prescription medications should ask their healthcare professionals before taking new medications or herbal remedies.
A pharmacist should be consulted when buying over-the-counter medications to be sure that they do not interact with prescription medications. A good way to help reduce the risk of drug interactions is to get all prescriptions filled at the same pharmacy. That way the pharmacist will have a complete record of all the medications the person is taking and check to make sure that there are no potentially dangerous interactions.
Cross-checking medications is especially important for people who are treated by more than one healthcare professional. Hundreds of possible drug interactions are known, and more are discovered every year. For this reason, it is wise for the clinician, the pharmacist, and the individual to check for possible drug interactions before any new substance is prescribed, dispensed, or taken.


Goodman and Gilman's Pharmacological Basis of Therapeutics, 1995, Hardman et al.

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