April 17 (HealthDay News) -- Low doses of an inexpensive drug called naltrexone -- already used for years to treat drug addiction -- helped reduce pain and fatigue in women with the painful disorder fibromyalgia, a new study has found.

"Physicians have been using this off-label for a while," said study co-author Jarred Younger, an instructor in anesthesia and pain medicine at Stanford University School of Medicine. The findings were published online April 17 in the journal Pain Medicine.

Fibromyalgia is a chronic ailment, marked by musculoskeletal pain and sensitivity to being touched. Some experts say it may affect up to 4 percent of the population.

Three prescription drugs are also on the market to treat fibromyalgia, Younger said. But, he added, "the three drugs don't solve the problem for everyone. Some don't respond to any of these drugs."

Younger said he got the idea to study naltrexone after hearing that some other doctors had success with it and after hearing from some members of fibromyalgia support groups that it had worked for them.

While the results of the small study look promising, Younger said it's too early to recommend the drug until more research is in.

In the study, he gave all 10 women a handheld computer and told them to enter daily reports of pain and other symptoms, describing them all on a scale of not bad to worst. The women were taken off their current fibromyalgia medication, then began to take placebo for two weeks (although they did not know it was placebo). Next they took naltrexone for 8 weeks. They then went through a two-week ''washout" period.

The researchers then evaluated their symptoms. "The drug reduced symptoms by 30 percent compared to placebo," Younger said. It was a statistically significant difference, he said.

That was an overall result, and the drug worked better for some than others. "Six out of the 10 were strong respondents, with at least a 50 percent reduction in symptoms," Younger said. It didn't work for all 10 women, he said, and some did not respond at all.

Naltrexone also seemed to ease certain symptoms more than others. "The drug helped daily pain -- the higher levels of pain, fatigue and stress," Younger said.

One symptom that did not typically improve was "fibro fog," the foggy thinking associated with the disorder, sleep quality, headaches, gastrointestinal problems and sadness.

Exactly how the drug works is not known for sure, Younger said. One possibility is that naltrexone quiets down a hyperactive immune system, he said.

Younger is continuing the research, and if the good results seen here bear out, naltrexone may be a good alternative for some patients. "It's a small dose, and we saw very few side effects," he said. The most typical was vivid dreaming.

Naltrexone has another point in its favor: cost. Doses of the drug, which must be gotten from special "compounding" pharmacies, total about $40 a month, Younger said.

Another expert called the study "fascinating."

"I was pleased to see that they had such a nice response to the therapy," said Dr. Patrick Wood, a family medicine physician with a specialty practice in fibromyalgia in Renton, Wash. He serves as medical adviser for the National Fibromyalgia Association.

The study was small, he said, but scientifically sound. If more research is promising, naltrexone may eventually be a competitor to higher-priced fibromyalgia drugs now on the market, Wood said.

SOURCES: Jarred Younger, Ph.D., instructor, anesthesia and pain medicine, Stanford University School of Medicine, Stanford, Calif; Patrick Wood, M.D., senior medical adviser, National Fibromyalgia Association, and family medicine physician with a specialty practice in fibromyalgia, Renton, Wash.; April 17, 2009, Pain Medicine, online