Related topics: •  Drug Information Center •  Herb-Drug Interactions

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 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.

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).

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

•Aspirin. This common, over-the-counter pain reliever also thins the blood. Aspirin may cause internal bleeding if combined with a prescription blood thinner such as warfarin (Coumadin). It can also decrease the effectiveness of some gout medications and increase the strength of certain diabetes drugs.

•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, certain antibiotics can hamper the effectiveness birth control pills and greatly increase the effects of warfarin. 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 send blood pressure soaring. Fluoxetine and similar drugs can also clash with St. John's wort and migraine drugs known as triptans. 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 even coma.

•Bronchodilators. The popular drug albuterol (Proventil, Ventolin) can cause dangerous spikes in blood pressure if combined with MAOIs or tricyclic antidepressants such as nortriptyline (Aventyl, Pamelor). If albuterol is combined with a beta blocker (used to treat blood pressure), neither drug will work as well as it should.

•Diabetes medications. Many drugs can block the effectiveness of glipizide (Glucotrol) and glyburide (Diabeta, Glynase, Micronase). Potential culprits include corticosteroids, hormones, diuretics, and antipsychotics. Several other drugs make the effects of glipizide and glyburide stronger, including blood thinners, insulin, MAOIs, aspirin, and the gout medicine probenecid (Benemid).

•Heart medications. The common heart drug digoxin (Lanoxin) can lose effectiveness if combined with antacids such as Mylanta or Maalox. On the other hand, its effects can be amplified by several other drugs, including diazepam (Valium) and 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 (Tenoretic, Tenormin) and reserpine (Serpalan, Serpasil-- another blood pressure medication sometimes prescribed to calm severe agitation) can cause a slow heartbeat and lower than normal blood pressure.

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).

Yes. Food can drastically change with the way drugs act in the body. Grapefruit juice, for example, blocks enzymes that metabolize certain drugs like calcium channel blockers, 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. 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.

-- Chris Woolston, MS, is a health and medical writer with a master's degree in biology, and the co-author of Generation Extra Large: Rescuing Our Children from the Epidemic of Obesity (Perseus, 2005). He is a contributing editor at Consumer Health Interactive, and was a staff writer at the late Hippocrates, a national magazine for physicians. He has also covered science issues for Time Inc. Health, WebMD, and the Chronicle of Higher Education.

Further Resources

Medical Letter's Adverse Drug Interaction Program

One of the best and most affordable programs, this can be ordered at 800-211-2769.

References

First published July 8, 2003
Last updated August 31, 2006
Copyright © 2003 Consumer Health Interactive

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