A Pfizer Inc. drug won expanded federal approval Thursday as the first treatment for fibromyalgia, a mysterious syndrome marked by muscle pain and fatigue.

The Food and Drug Administration action means Lyrica becomes the first drug that can be marketed specifically as a treatment for adults with fibromyalgia. Patients currently make so-called "off label" use of pain medications, antidepressants, muscle relaxants and sleep aids to treat the condition. Exercise and applying heat also can help.

Fibromyalgia typically affects women, striking them with long-lasting or chronic pain, as well as muscle stiffness and tenderness, according to the FDA. An estimated 3 million to 6 million people in the United States are affected each year.

Lyrica, known generically as pregabalin, previously won FDA approval to treat partial seizures, pain following the rash of shingles and pain associated with diabetes nerve damage.

The FDA warned that common side effects of the drug included mild-to-moderate dizziness and sleepiness. Also, the agency said that Lyrica reduces pain and improves daily functions for some patients with fibromyalgia, but that not everyone derived benefit from the drug in studies.

The cause of fibromyalgia is a mystery, though it may be linked to injury, emotional distress or viruses that change the way the brain perceives pain, the FDA said.