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Drug Interactions: What You Need to Know

You might know that certain prescription medications can interact with one another and cause potentially harmful side effects. But did you know that interactions can occur not only with prescription medications, but also with over-the-counter medications, supplements, and foods and beverages? Medications can even interact with diseases or conditions you may have. Fortunately, with a little careful planning, you can avoid serious drug interactions.

Types of Drug Interactions

There are 3 basic types of drug interactions:

Drug-Drug Interactions

These occur when one drug interferes with another drug, affecting how one or both act in or are eliminated from the body. These interactions can occur between prescription drugs, over-the-counter drugs, and even herbal or other dietary supplements, including vitamins. For example, vitamin E and aspirin both act to thin the blood. Taking these together could cause excessive bleeding. And combining antidepressants with the pain medication tramadol could cause seizures.
It is particularly important to remember that herbal products, which many people regard as natural alternatives to drugs, still behave like drugs in the body. For example, the herb called St. John’s wort can reduce blood levels of certain medications. Furthermore, if a person is already taking St. John’s wort along with another drug, stopping the herb may cause drug levels to rise, potentially leading to dangerous complications.

Drug-Food/Beverage Interactions

Drug-food/beverage interactions occur when a prescription or over-the-counter medication interacts with food or beverages. For example, taking the antibiotic tetracycline with a glass of milk can lessen the absorption of the antibiotic in the body and make it less effective. Grapefruit juice can block enzymes that metabolize numerous drugs, including some blood pressure-lowering drugs, anti-depressants, antihistamines, and the drug cyclosporine, thereby increasing blood levels of these drugs. Toxicity could result.

Drug-Condition Interactions

These occur when a prescription or over-the-counter medication interacts with a disease or condition. For example, decongestants, such as those found in many over-the-counter cold remedies, can cause an increase in blood pressure, which could be dangerous for people who already have high blood pressure.

Symptoms of Drug Interactions

The most common symptoms of drug interactions tend to be less serious and include the following:

More serious—but less common—symptoms and results of drug interactions include the following:

Consult your doctor or pharmacist if you experience any unusual side effect after taking a medication, no matter how mild or severe.

How to Avoid Drug Interactions

The key to avoiding drug interactions is to become informed about the potential interactions between all the drugs and dietary supplements you take by talking with your doctor and pharmacist.

Some steps you can take include:


United States Pharmacopeia (USP)

US Food & Drug Administration


Canadian Medical Association

Health Canada


Drug interactions: what you should know. US Food & Drug Administration website. Available at: Updated September 25, 2013. Accessed May 1, 2017.

Avoiding drug interactions. US Food & Drug Administration website. Available at: Updated July 25, 2015. Accessed May 1, 2017.

Mallet L, Spinewine A, Huang A. The challenge of managing drug interactions in elderly people. Lancet. 2007;370:185-191.

Neuvonen PJ. Interactions with the absorption of tetracyclines. Drugs. 1976;11(1):45-54.

Sansone R, Sansone L. Tramadol. Psychiatry. 2009;6(4):17-21.

St. John's wort. EBSCO Natural and Alternative Treatments website. Available at: Updated December 15, 2015. Accessed May 1, 2017.

Last reviewed April 2017 by Michael Woods, MD, FAAP  Last Updated: 5/1/2017