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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? 
Drug interactions 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, and vitamins. Homeopathic remedies and recreational drugs, such as alcohol, tobacco, and illegal drugs, can also interact.

Effects of drug 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.
  • 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 contraceptive pills to be broken down too quickly. A woman taking both of these medications risks becoming pregnant.
  • 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.

How drugs interact

There are several ways in which drug interactions can take place. 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 interact at the cell level. For example, one drug may push the other drug off the cell and make it less effective. There are a number of other kinds of interactions.


When a new medication is prescribed, a person should tell his or her doctor what other drugs he or she is taking. To avoid serious drug interactions, it's important to mention medications that have been prescribed by other doctors. A person cannot afford to be quiet about taking birth control pills, over-the-counter medications, herbal remedies and complementary therapies, or illegal drugs. All drugs and substances are important to mention.

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 doctors before taking new medications or herbal remedies. A pharmacist can 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. The pharmacist can make sure there are no potentially dangerous interactions. This is especially important for people who are treated by more than one doctor.

There are hundreds of possible drug interactions. The list grows longer every year. It's wise to check for possible drug interactions before any new substance is taken.

Author: Adam Brochert, MD
Reviewer: eknowhow Medical Review Panel
Editor: Dr John Hearne
Last Updated: 7/02/2005
Potential conflict of interest information for reviewers available on request

This website and article is not a substitute for independent professional advice. Nothing contained in this website is intended to be used as medical advice and it is not intended to be used to diagnose, treat, cure or prevent any disease, nor should it be used for therapeutic purposes or as a substitute for your own health professional's advice.  All Health and any associated parties do not accept any liability for any injury, loss or damage incurred by use of or reliance on the information.


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