Longtime sufferers of seasonal depression may now have a way to stop the winter blues before they start. The Food and Drug Administration (FDA) announced Monday that it has approved the prescription medication Wellbutrin XL to prevent major depressive episodes caused by seasonal affective disorder. The antidepressant is the first drug approved to treat the condition, which is estimated to affect as many as 10 million Americans each year.

In these people, low levels of sunlight during autumn and winter can trigger a depression that may last as long as six months. Common symptoms include fatigue, oversleeping, impaired social functioning, and weight gain. Doctors often use antidepressant medications and light therapy to treat seasonal affective disorder; now, GlaxoSmithKline, the manufacturer of Wellbutrin XL, will be able to market the drug expressly as a preventive measure that can be taken in the fall to head off symptoms.

Many people experience only mild impairment from seasonal affective disorder and don't need medication, notes Douglas Jacobs, associate clinical professor of psychiatry at Harvard Medical School. But there is "a subset of people who suffer severe symptoms on an annual basis," he says. For them, "this is a medication that can be very, very useful." (Jacobs has no direct affiliation with GlaxoSmithKline, although he heads a nonprofit organization dedicated to mental health screening that receives an educational grant from the company.)

But some doctors are not convinced the medication's benefits are so clear. GlaxoSmithKline tested 1,024 patients with a history of seasonal affective disorder and found that, while 84 percent taking Wellbutrin XL were able to avoid a depressive episode, so were 72 percent of the people taking a placebo. "That is an enormously high placebo rate," says Dr. Michael Terman, director of the Center for Light Treatment and Biological Rhythms at Columbia University Medical Center. Terman, who worked on the Columbia University arm of the multicenter trial, points out that a number of effective medications can be used to lift the depression once it hits.

But Norman Rosenthal, medical director of Capital Clinical Research Associates in Rockville, Md., and an investigator on the study speaking on behalf of GlaxoSmithKline, points out that the high placebo rate might be attributed to the high level of attention people received while participating in the trial. "Some of the people who might otherwise feel depressed feel supported," he says.

Terman worries about the wisdom of prescribing a powerful drug to people without symptoms, especially given that "not every single winter is going to produce a major depression." He cautions: "Wellbutrin has significant side effects that are not present in a first-line intervention like light therapy." According to the manufacturer, the side effects include weight loss, seizure, dry mouth, shakiness, stomach pain, anxiety, difficulty sleeping, muscle pain, and nausea. The drug's label also contains a "black box" warning that the pill can cause an increased risk of suicidal thoughts and behavior in children and adolescents.

"You have to weigh the risks of being depressed against the side effects of the drug," says Rosenthal. "You have to make the calculation together with your doctor. The FDA says that is a very reasonable thing for you and your doctor to be doing."

Learn more about depression in our online guide.