All drugs carry the risk of side effects—some so serious that they're flagged for doctors in so-called black-box warnings required by the Food and Drug Administration. But a study just published in the journal Pharmacoepidemiology and Drug Safety found that doctors don't always heed the drug makers' guidance.

Researchers looked at 930,000 patients from across the country and found that more than 40 percent were prescribed at least one drug with a black-box warning during the 2½-year study period. But of those people prescribed valproic acid for epilepsy, for example, some 30 percent weren't getting routine blood tests to test for liver damage as the warning advises.

In many cases, rushed doctors undoubtedly miss the warnings entirely. But there also can be good reason to ignore them, says study author Anita Wagner, a health policy researcher at Harvard Medical School. Many doctors feel that routine liver testing is an unnecessary burden for patients who have been taking valproic acid for years without trouble, for example.

Clinical medicine is fluid, says Donna Sweet, chair of the board of regents for the American College of Physicians.

"We all do things in our day jobs that the package insert doesn't say whether you should or shouldn't do. We use [a drug] for one thing, and yet it isn't indicated by the FDA for that thing." And not all black-box warnings are equal in import. Sweet was pleasantly surprised that doctors in the study almost never gave pregnant women drugs such as the acne medication Accutane that can hurt their babies.

Still, says Sweet, the fact that doctors do quite often ignore the guidance suggests that the black-box warnings aren't doing a good enough job of getting the word out about drug risks. So patients should check out the drugs they take and ask their doctors how to address any warnings. A wide variety of medications are affected, from oral contraceptives (don't smoke while you're taking them) to beta blockers (don't stop taking them abruptly) to the AIDS drug nevirapine (unsafe for women with high levels of certain immune cells).

You can ask your pharmacist if warnings have been issued about your drugs, or try your luck online.

Wagner says she hasn't been able to find any complete, regularly updated list of drugs with warnings. Try searching the FDA's MedWatch for information on specific drugs.

The National Library of Medicine is a good place to start for drug information, but black-box warnings are not always highlighted.

You can also look in the most recent Physician's Drug Reference at the local library. This giant volume contains the text of package inserts included with thousands of drugs. (Package inserts are for healthcare providers and contain much more detailed information than the papers given to patients with their drugs—including any black-box warnings.) This page from epilepsy.com (http://www.epilepsy.com/epilepsy/medicine_insert.html) has advice on how to read a package insert.