June 16 (HealthDay News) -- Eli Lilly said Monday that its antidepressant Cymbalta (duloxetine) has been approved by the U.S. Food and Drug Administration to treat fibromyalgia, a chronic disorder with symptoms including widespread muscle pain and tenderness.

The condition affects about 2 percent of the American population, or about 5 million people, mostly women. While its cause is unknown and there is no known cure, it's believed it may be related to a combination of changes in brain and spinal cord chemistry, genetic factors, and stress, the company said in a statement.

Cymbalta affects production of two naturally occurring brain substances, serotonin and norepinephrine. In addition to affecting mood, it's believed these substances are part of the body's natural pain-surpressing system, Lilly said.

In a pair of three-month trials involving 874 people with fibromyalgia, Cymbalta significantly reduced pain levels, compared with a non-medicinal placebo, the company said. Common adverse reactions included nausea, dry mouth, constipation, decreased appetite, and sleepiness.

Cymbalta also is approved to treat major depressive disorder and generalized anxiety disorder, and a form of nerve pain in diabetics, all in adults 18 and older.