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By Lewis Krauskopf

NEW YORK (Reuters) - Not all doctors are sure about the pain and fatigue condition known as fibromyalgia, but drug companies are racing to win U.S. regulatory approval to serve this potentially lucrative market.

The sometimes-debilitating disorder afflicts an estimated 2 percent to 4 percent of Americans, mainly women.

But diagnosing fibromyalgia is not easy because its cause remains unknown and its symptoms, which also include depression, can overlap with other conditions.

With no test to confirm fibromyalgia, doctors rely on patient complaints of symptoms and subjective responses to physical exams. As a result, some physicians are wary of viewing it as a distinct ailment.

Still, a who's who of pharmaceutical companies -- including Pfizer Inc., Eli Lilly & Co., Forest Laboratories Inc. and Wyeth -- are looking to seize on a market now dominated by older anti-depressants as well as painkillers and other drugs.

"What they're thinking is: This has a huge, untapped, unmet need," said Maria Marzilli, an associate analyst with market research firm Decision Resources.

Decision Resources expects sales for drugs used for fibromyalgia to roughly triple to at least $1 billion by 2014

The companies are vying for the first clearance by the U.S. Food and Drug Administration for a fibromyalgia treatment, which could occur as soon as 2008.

Doctors can prescribe medicines for fibromyalgia even though the drugs are not cleared specifically for it. However, without FDA approval, companies cannot promote the drugs as treatments for that condition.

Therefore, even though doctors already may be prescribing Pfizer's Lyrica and Lilly's Cymbalta for fibromyalgia, positive clinical data and U.S. regulatory approval for that use could jump-start sales.

"This is a nice way to tack on $200 to $300 million more in sales," Marzilli said.

An FDA approval could also quell doubts about the disorder's legitimacy, while paving the way for companies to promote medicines specifically for fibromyalgia. To be sure, doctors and consumers will hear more about the condition as the drug makers rev up their powerful marketing engines, possibly sparking more widespread diagnosis.

"If the FDA approved a drug for fibromyalgia, that would really give the field validity," said Richard Harris, a molecular biologist and research investigator at the University of Michigan who recently published a data review of fibromyalgia.

In most cases, the companies are conducting clinical trials in fibromyalgia for drugs already approved for other conditions. Lilly, Forest and Wyeth have said they are testing anti-depressants that regulate two brain transmitters, serotonin and norepinephrine. Pfizer is testing a drug cleared for epileptic seizures and neuropathic pain.

As classified by the American College of Rheumatology in 1990, a fibromyalgia diagnosis involves a patient feeling muscle tenderness in at least 11 of 18 predetermined sites on the body, with the pain spread throughout the body.

Fibromyalgia patients also tend to tire easily, struggle to sleep and have trouble remembering things, a problem referred to as "fibro fog."

But patients often receive other diagnoses before their doctors decide they have fibromyalgia.

Lynne Matallana, founder and president of the Orange, California-based National Fibromyalgia Association, was an advertising executive and avid skier and bicyclist in the early 1990s until she began experiencing widespread pain and fatigue that left her bedridden.

She said she had seen 37 doctors and received diagnoses of lupus, rheumatoid arthritis and depression before a physician finally identified her condition as fibromyalgia.

"It's devastating because it is totally life-altering, and many, many, many people find themselves in this situation," Matallana said. She added that she gradually improved through exercise, meditation and medication.

In their review, Harris and a colleague at the University of Michigan reported evidence of increased neurological responses to pain, indicating that the central nervous system of a fibromyalgia patient processes pain differently

Traumatic events -- such as a car accident -- may trigger the condition. Matallana said her fibromyalgia flared up after she underwent surgery for another ailment.

"I think that some physicians do believe that fibromyalgia isn't a real condition, and our job to spread the word that there are objective findings that these people are in pain," Harris said.

But Dr. Scott Zashin, a rheumatologist in private practice in Dallas, says fibromyalgia is one of the more common conditions he sees.

Zashin says he usually tries to get fibromyalgia patients to exercise more or get more rest before turning to medication.

"These patients seem to have an increased sensitivity to pain," Zashin said. "Something in their makeup makes them experience pain differently."