GlaxoSmithKline’s antidepressant Wellbutrin XL has become the first prescription drug to be approved for the prevention of depression caused by seasonal affective disorder (SAD), a mood disorder thought to be caused by seasonal decreases in daylight.

Although the exact number of Americans suffering from SAD is not known, it is believed that up to 10% of the US population may suffer from mild symptoms associated with the disorder, depending on their geographic location, according to the American Psychiatric Association.

Major depressive symptoms occur in around 6% of the population, according to GSK estimates. At the moment, SAD tends to be treated using fluorescent light boxes that are designed to mimic exposure to sunlight, although there is evidence of some off-label use of antidepressants in severe cases, which may have tailed off in light of the link between antidepressants and suicide in young people. SAD affects younger people more commonly than those of more advanced years.

A combined analysis of three clinical trials showed that Wellbutrin XL (bupropion) reduces the chance of developing a seasonal major depressive episode by 44%, said GSK. Meanwhile, 84% of people who took Wellbutrin XL between September and March were depression-free, versus 72% among those who took a non-medicinal placebo, according to the FDA.

“Seasonal affective disorder can significantly impair the quality of life of patients with this condition,” said Dr. Steven Galson, Director for FDA’s Center of Drugs and Research.

He noted that Wellbutrin XL is only indicated for patients who meet “strict diagnostic criteria of seasonal major depressive episodes. Such patients have a pattern of recurrent, clinically significant depressive symptoms with associated impairment of functioning.”

“The clinician and patient should carefully assess the potential risks and benefits when considering treatment with Wellbutrin XL for SAD,” said Galson.