Harvard Women's Health Watch

Making sense of side effects

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The package inserts don't give a good idea
of which side effects you are likely to have.

Any drug you take can cause a new set of problems, but you may be able to anticipate and minimize them.

If you've ever taken a drug and felt even worse after you started taking it, you may have wondered what was going on. Dr. Gordon Schiff, associate professor of medicine at Harvard Medical School, has a simple explanation: "All drugs have effects—the ones we want and the ones we don't. The unwanted effects are known as side effects."

Why drugs have side effects

Nearly all medicines have many effects on your body. When you take a drug, it is distributed throughout your organs and tissues, and it may do different things in different systems. Take aspirin, for example. Aspirin has been around for more than a century, so there is a lot of information on what it does. Aspirin's anti-inflammatory action may suppress your arthritis, but it also inhibits the body's production of the chemicals that protect your stomach lining and help your blood clot. As a result, indigestion and bleeding are common side effects of aspirin use.

But the side effects of newer drugs are less well known. For many products, information on side effects is gleaned in the clinical trials required for approval by the FDA. During such studies, researchers note the effects that occur in people taking the study drug and compare them to the effects in people taking a placebo. That information includes every side effect experienced by every person who was taking the drug during the study period.

Once a drug is on the market, its manufacturer is required to inform physicians and consumers of those side effects, resulting in the long lists printed in microscopic type in magazine and newspaper ads and intoned on television commercials. For over-the-counter drugs, the list is included in the box or printed on the label. Most of these lists are hard to read not only because they require a magnifying glass but also because they are written in medicalese. If you've ever tried to make sense of the list, you've probably heard your inner voice screaming, "Too much information!"

Dr. Schiff sympathizes. "Listing scores and scores of side effects doesn't help you sort out what is most likely to happen to you," he says. However, there is one bit of information that's essential to have, Dr. Schiff adds. It's the "black box" warning, and it's usually written in plain English. Drugs that have side effects that may lead to death or serious injury will have this warning information displayed prominently in a box in the prescribing information.

Reducing your chance of side effects

Dr. Schiff suggests a few things you can do to protect yourself.

  • Ask for a drug that's been on the market a while. The information on side effects of a newly released medication is often based on clinical trials involving a few hundred people, but an older drug is likely to have been used in hundreds of thousands of people. That experience can reveal additional side effects and give doctors an idea of which side effects are most common, which are most serious, and which might occur only after months or years of use.

  • Talk to your doctor about what to expect. Ask what side effects his or her patients have experienced, how severe they were, and what you can do if you experience them. For example, if nausea is a potential side effect, you'll want to know if you should keep taking the drug because the nausea will eventually go away or if you should stop taking it. If you're taking a benzodiazepine or an opiate, you'll want to know about the side effects of withdrawing from the drug and develop a plan for tapering off.

  • Listen to your body. If you develop a symptom that might be a consequence of using a new drug, let your physician know. It's the best way to tell whether you're experiencing a side effect.

  • Don't stop taking a new medication without talking to your doctor first. Your doctor should review with you the most common or worrisome side effects you might experience. If, after starting a new medication, you read about an effect that your doctor did not discuss but that worries you, don't just stop taking the new drug. Call your doctor to discuss your concerns first.

Reporting side effects

You can report your side effects online to the FDA at health.harvard.edu/medwatch. To report a side effect from a vaccine, go to health.harvard.edu/vaccine-problem.