Eli Lilly is celebrating the news that US regulators have given their approval for the firm's blockbuster antidepressant Cymbalta to treat fibromyalgia.

Cymbalta (duloxetine), currently approved in the USA for adults with major depressive disorder, generalised anxiety disorder and the management of diabetic peripheral neuropathic pain, has been approved for fibromyalgia on the basis two pivotal three-month clinical trials involving 874 patients. In both studies, Cymbalta reduced pain at study endpoint compared with placebo as measured by the Brief Pain Inventory (BPI) 24-hour average pain scale. Significant improvement in pain for the Lilly drug versus placebo was observed in the first week of each study and 51% and 55% of patients on Cymbalta had a 30%improvement on the BPI at endpoint.

The cause of fibromyalgia remains unknown but scientists believe it may be related to "some combination of changes in brain and spinal cord chemistry, genetics and stress", Lilly said. Some researchers believe it is a disorder of increased sensitivity to pain. and although the way Cymbalta works in people is not fully known, medical experts believe it increases the activity of two naturally occurring substances – serotonin and norepinephrine. Fibromyalgia is estimated to affect 2%of the US population - approximately 5 million people - the majority of those diagnosed being women.

Now that Cymbalta has been given the green light for fibromyalgia, it will go up against Pfizer’s Lyrica (pregabalin), which was approved by the US Food and Drug Administration in June 2007, while future competition could come from Wyeth’s Pristiq (desvenlafaxine) and UCB’s Xyrem (sodium oxybate). The approval will give a boost to the drug's already-considerable sales which reached $605.1 million in the first quarter, up 37%.