Eli Lilly and Co. said Monday it received Food and Drug Administration approval to expand the use of its fastest-growing drug, the antidepressant Cymbalta.

Regulators approved Cymbalta to treat fibromyalgia, a chronic pain disorder. The drug already is approved for diabetic nerve pain, major depressive disorder and generalized anxiety disorder.

Lilly launched Cymbalta in 2004. It was the drugmaker's second-best seller last year, generating $2.1 billion in mostly U.S. sales behind Zyprexa's $4.7 billion. Cymbalta sales rose 37 percent in the first quarter to $605 million.

Cymbalta's 2007 sales represented a 60 percent increase over 2006. That year, the drug notched $1.3 billion in sales, a 94 percent increase over 2005.

Fibromyalgia affects about 5 million Americans. Researchers believe its cause may be related to genetics, stress and changes in brain and spinal cord chemistry and that it leads to increased pain sensitivity.