A widely used seizure drug may help treat fibromyalgia, a condition that causes muscle pain, fatigue and depression, a U.S. government-sponsored study found.

Patients taking the medication, gabapentin, had fewer aches, felt less tired and slept better, according to results released Monday by the National Institutes of Health, the sponsoring agency. Depression wasn't eased. The drug is sold by Pfizer Inc. as Neurontin and by a dozen other companies in generic form,

Fibromyalgia affects 6 million Americans, mostly women. It is usually treated with drugs for other illnesses, including antidepressants and analgesics such as aspirin. While researchers say the condition is poorly understood, they suspect it occurs when the brain is overly sensitive to pain signals. Gabapentin may help by causing fewer molecules that process those signals to be released, according to the scientists.

"While gabapentin does not have Food and Drug Administration approval for fibromyalgia, I believe this study offers additional insight to physicians considering the drug for their fibromyalgia patients," said Stephen Katz, the director of the National Institute of Arthritis and Musculoskeletal and Skin Disorders, part of the NIH.

Doctors may prescribe drugs for uses that haven't received FDA approval, although drugmakers may not promote those uses.

In the 150-patient study, led by Lesley Arnold at the University of Cincinnati, 51 percent of those on the drug saw a drop in pain of at least 30 percent. That much reduction was seen in 31 percent of patients taking a placebo.

Lyrica

Cheaper copies of Neurontin came on the market in 2004 when the drug lost patent protection. Pfizer is seeking U.S. clearance to market a newer, more-expensive drug called Lyrica, which works similarly to Neurontin, for fibromyalgia.

A Pfizer spokeswoman, Shreya Prudlo, said the company isn't planning to conduct the required studies to get Neurontin approved by U.S. regulators for treating fibromyalgia.

In a separate study of Lyrica on fibromyalgia, 30 percent of patients saw their pain cut by more than half, a reduction seen in 15 percent of the people taking a placebo, New York-based Pfizer said last month. Lyrica is approved to treat pain from shingles and diabetes.

Pfizer says Lyrica has fewer noticeable side effects than Neurontin, which caused diarrhea, gas and other digestive disorders. Neurontin was also a much larger pill taken more frequently at higher does.

Pfizer shares fell 15 cents, or less than a percent, to $26.37 at 4 p.m. in New York Stock Exchange composite trading. The shares have gained 12 percent in 12 months.