Why and How to Check for Drug Interactions

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Definition of drug interaction

If you take any drugs, whether they are prescription or over-the-counter, you should be aware of the potential for drug interactions. A drug interaction occurs when the combination of a drug and another substance affects the activity of the drug. If the substance is another drug, then it is a drug-drug interaction). For example, amlodipine (for high blood pressure) increases the level of simvastatin (for high cholesterol) in the blood when both drugs are taken together.

Excessively high levels of simvastatin can cause liver and muscle damage.

If the substance is a food, then it is a drug-food interaction. Grapefruit juice increases the absorption of many drugs into the bloodstream, potentially increasing the drug concentrations to dangerous levels.

Major types of drug-drug interactions include:

  • One drug affects the way your body eliminates (gets rid of) another drug
  • One drug affects the way your body metabolizes (makes alterations to) another drug
  • One drug affects the way your body absorbs another drug
  • One drug affects the distribution of another drug within the body
  • Both drugs cause the same effects

[Read more about drug interactions.]

How to check for drug interactions

Systems are in place to reduce the likelihood of a drug-drug interaction. Your health care provider may use an electronic health record (EHRs) system to send electronic prescriptions to your pharmacy.

Some of these EHRs have drug-drug interaction alerts to warn the prescriber of a potential interaction. Pharmacists also have computerized systems to check for drug interactions. They can discuss potential interactions with the patient or the prescriber. But neither of these safety nets can prevent all drug interactions.

The doctor or pharmacist may ignore the alert, or your record might not list all the medications you are taking. (That's why it's vital to make sure all of your health care providers are aware of all the drugs you are using, including prescription medications, over-the-counter medications, vitamins, herbal remedies, and food supplements.)

You can take an active role in making sure your medication combinations are safe by using online drug interaction checkers. The following list describes important features of a few drug interaction checkers, all of which are free and designed for patients.

FeatureDrugs.comRxList.com Medscape.com

Drop-down menu allows you to select

the precise drug name


Can check interactions for more

than 2 drugs at a time

Describes the severity of interactionYesYesYes

Includes information on drug-food 




You'll get a more comprehensive view if you check multiple sources. Some variation is inevitable due to differences in the underlying drug database from which the interaction checkers draw their information.

For example, 

What to do if your drugs interact

A drug-drug interaction doesn't always mean that you have to stop taking one of the drugs. If you do find that a certain drug combination you are taking would cause interactions, then talk to a trusted health care provider -- preferably the one who prescribed the drugs in the first place. They can discuss with you whether the risk of the interaction outweighs the benefits of taking the drugs. Make it a habit to check for drug interactions every time you start taking a new medication.

[Mention of a commercial product or service does not constitute an endorsement.]

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