Drug Interactions

Drug Interactions
By Chris Woolston, M.S.

Remember those high school chemistry experiments in which you mixed two harmless chemicals and got a bizarre reaction? You may be performing a similar experiment on yourself every time you take two medications at the same time. Certain drugs react strongly when taken with others, often causing serious side effects. In rare cases, drug interactions can even be deadly.

Drugs can affect each other in many different ways. One medication can often block or enhance the action of another. For example, common over-the-counter antacids can reduce the effectiveness of some antibiotics (see below). In other cases, certain drugs may work too well together. For instance, both aspirin and the prescription drug warfarin (Coumadin) can help prevent blood clots by thinning the blood. In combination, however, they can raise the risk of serious bleeding.

What should I be aware of?

Fortunately, you can take steps to protect yourself from dangerous drug interactions. First and foremost, make sure all of your doctors know about every medication you currently take, including over-the-counter drugs and herbal remedies. Ideally, you should bring a list of your medications to every doctor’s appointment. Better still, bring all of the medications you take to the doctor so that you can discuss them. You should also carefully read the labels of all your medications. If you have any questions, don’t hesitate to ask your doctor or pharmacist for help.

You may also want to do some research on potential interactions beforehand, so you’ll be more prepared when you meet with your doctor or pharmacist. All prescriptions come with a package insert that describes the strength, dosage, and side effects of the drug you’re taking. But if you’ve lost yours or already thrown it away, you can find the package insert for your medication online, or ask your pharmacist for a copy of it. Read over the sections on potential side effects and drug interactions carefully. (Versions of some package inserts are available for both consumers and medical professionals; the handout for doctors is more complete, so you may want to ask for a copy of both).

What are some drug combinations I should avoid?

Here are some examples of common drugs that can clash with other medications.

  • Aspirin. This common, over-the-counter pain reliever also keeps the platelets in your blood from sticking together, increasing your risk of bleeding. Taking aspirin with a prescription blood thinner such as warfarin (Coumadin) can significantly increase this risk. In some cases your doctor may want you to take both, but will monitor the clotting action of your blood with frequent blood tests. Aspirin should not be taken with certain gout medications because it can change uric acid levels in the blood and make a gout attack worse.
  • Antibiotics. Some forms of these infection-fighting drugs can lose their power if combined with antacids (i.e., Mylanta, Maalox, Tums) or other products containing calcium. In addition, the antibiotic rifampin has been found to decrease the effectiveness of birth control pills. Other antibiotics may have a similar effect, but more studies are needed. Probenecid (Benemid), a drug used to treat gout, can increase blood levels of several different types of antibiotics. In some cases, doctors may even use this interaction to their advantage: For extra punch against germs, doctors sometimes prescribe this drug along with antibiotics.
  • Antidepressants. Newer antidepressants known as SSRIs, such as fluoxetine (Prozac) and paroxetine (Paxil), shouldn’t be mixed with older mood-lifters known as MAOIs (such as phenelzine). This combination can cause serotonin syndrome, a potentially life-threatening condition characterized by muscle rigidity, fever, high blood pressure and heart rate, confusion, and even coma. Fluoxetine and similar drugs can also clash with St. John’s wort to cause similar symptoms. Potential side effects of such combinations include confusion, fever, high blood pressure, and tremors. Another class of antidepressants called tricyclics (such as Elavil), can also clash with MAOIs, causing confusing dizziness, seizures, and coma.
  • Bronchodilators. The popular drug albuterol (Proventil HFA, Ventolin HFA) can cause dangerous spikes in blood pressure if combined with MAOIs or tricyclic antidepressants such as nortriptyline (Aventyl, Pamelor).
  • Heart medications. The common heart drug digoxin (Lanoxin) can lose effectiveness if combined with antacids such as Maalox. On the other hand, its effects can be amplified by several other drugs, including diazepam (Valium) and some antiarrhythmia medications. Heart drugs known as nitrates can trigger dangerously low blood pressure if taken with the erectile dysfunction drug sildenafil (Viagra). The combination of the blood pressure medication atenolol (Tenormin) and some calcium channel blockers (especially verapamil) can cause a potentially dangerous reduction in heart rate.
  • Statins. A scientific statement from the American Heart Association says that prescribing these cholesterol-lowering drugs with a number of heart medications can cause dangerous drug interactions. Published in the journal Circulation, the statement reported the following drugs could cause problems when taken with statins:
  • Fibrates also used for cholesterol lowering
  • Calcium channel blockers
  • Blood thinners
  • Antiarrhythmic drugs
  • Heart failure medications
  • Immunosuppressive agents

Prescription drugs and over-the-counter medications aren’t the only worry. Drugs can also interact with herbal remedies such as ginkgo biloba, which inhibits blood clotting. Ginkgo, among other herbs, can increase the risk of dangerous bleeding if it’s taken with an anticoagulant such as Coumadin (warfarin).

Can food affect the way drugs work?

Yes. Food can change the way drugs act in the body. Grapefruit juice, for example, blocks enzymes that metabolize certain drugs like calcium channel blockers and statins, leaving higher levels of medication in the bloodstream.

If you drink alcohol regularly, you may also need to be careful about taking over-the-counter pain relievers. Having three or more alcohol drinks each day puts you at risk for liver damage if you take acetaminophen (Tylenol), and taking aspirin, ibuprofen, and naproxen with alcohol can lead to peptic ulcers and other stomach problems.

Still, drug-to-drug interactions are the leading concern. Keep in mind that the list above only scratches the surface of potentially dangerous drug interactions.

You may want to use online tools to check the potential for interactions between food you eat and the medicines you take. Websites for seniors like the AARP’s and those from hospitals also provide online drug checkers that can tell you about potential interactions.

There are many other potential hazards out there. Talk to your doctor about the drugs in your medicine cabinet. You just may discover that they don’t always get along.


AARP. Drug Interaction Checker. http://healthtools.aarp.org/drug-interactions

Guide Helps Pharmacists Be More Aware of Statin, Cardiac Drug Interactions, Nov.2, 2016. https://www.uspharmacist.com/article/guide-helps-pharmacists-be-more-aware-of-statin-cardiac-drug-interactions

Cleveland Clinic. Do Your Statins and Grapefruit Safely Mix? https://health.clevelandclinic.org/2016/01/statins-grapefruit-safely-mix/

U.S. Food and Drug Administration. Drug Interactions: What you should know.

The Columbia University College of Physicians and Surgeons. Complete Home Medical Guide. Appendix A: Commonly Prescribed Drugs.

Time Inc Health. The Self-Care Advisor.

Talking With Your Doctor, National Institutes of Health

Rott, KT, Agudelo, CA. Gout. Journal of the American Medical Association 289(21): 2857-60.

Mayo Clinic. Birth Control Pills FAQ: Benefits, Risks and Choices.

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Mayo Clinic. Daily Aspirin Therapy: Understand the Benefits and Risks.

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