The agency's decision endorses doctors' use of Botox to treat patients who suffer from a severe form of migraine involving headaches on at least 15 days a month. Britain's drug agency approvedBotox for the same use this summer.
Botox is already approved by the F.D.A. to treat uncontrolled blinking; crossed eyes; certain neck muscle spasms; excessive underarm sweating; and stiffness associated with muscle spasticity in the elbows and hands. It also is approved for cosmetic purposes — to smooth lines between the eyebrows.
Botox had worldwide sales last year of about $1.3 billion, divided equally between medical and cosmetic uses.
But Allergan said sales of Botox for chronic migraine and other medical uses would soon eclipse sales of the drug as a wrinkle smoother. Allergan is also studying the drug for a variety of new medical uses, including overactive bladder, said Dr. Scott M. Whitcup, the company's executive vice president for research and development.
"For the business, Botox has been an incredible medication. We call it our pipeline in a vial," Dr. Whitcup said. "People still think about it as a cosmetic product, but the therapeutic indications in the next five years will far surpass its cosmetic use."
Industry analysts have forecast worldwide sales of the drug for the severe migraine condition at $250 million to more than $1 billion annually by 2015.
Unlike the occasional headache, the chronic migraine condition is often accompanied by nausea, vomiting, dizziness, intense sensitivity to light and noise, and moderate to severe pain.
The audience for Botox headache shots could be significant because some chronic migraine patients do not improve when they take the pills that are now the standard treatment, neurologists said. Treatments include pills like Topamax, taken daily to prevent migraine, and the triptan family of drugs, taken to ease an existing migraine.
Botox is a purified form of botulinum toxin, a nerve poison produced by the bacteria that causes botulism. Injections of Botox typically act to temporarily blunt nerve signals to certain muscles or glands. Researchers are still exploring how the drug works on migraines. Dr. Whitcup said one theory was that it blocked pain signals from reaching nerve endings.
A Botox migraine treatment generally involves a total of 31 injections in seven areas — including the forehead, temples, the back of the head, the neck and shoulders. To treat the chronic condition, injections are given about every three months.
Industry analysts estimated that the migraine treatment would cost $1,000 to $2,000, depending on the amount of the drug used and the physician's fee. Some private insurers are likely to cover the migraine treatment now that it has received F.D.A. approval, analysts said, although patients may have to cover a significant co-payment.
"The cost is prohibitive for some," Randall Stanicky, a vice president for global research at Goldman Sachs, said in an interview earlier this year. "But given the debilitating challenges of having migraines more than 15 days a month, if Botox can cut down on that, it's clearly going to be a big opportunity."
Other analysts have expressed skepticism that doctors and patients would embrace the drug, arguing that Botox has a marginal effect on headaches compared with a placebo.
"The true drug effect is minimal," Corey Davis, an analyst at Jefferies & Company, said in an interview earlier this year.
Patients in one study financed by Allergan, for example, typically experienced about five fewer headache episodes a month than they had before the study — no matter whether they had injections of Botox or a placebo.
After Allergan reviewed the results of that first study, the company changed the primary end point — the scientific goal post — on a second study so that it would focus on the drug's effect on the number of headache days rather than the number of headache episodes that a person experienced each month. Dr. Whitcup said it was easier for patients to remember how many headache days as opposed to how many headache episodes they had every month.
The second study reported that patients who received Botox injections typically experienced about 2.3 fewer headache days than the placebo group, a statistically significant difference. But the placebo group also experienced considerable improvement — a common feature in pain studies — raising questions among some doctors about the magnitude of the Botox effect.
Dr. Whitcup said Botox had consistently beaten the placebo at different time points in the study and that patients had reported an improvement in their daily functioning and quality of life.
Although the F.D.A. approved the drug for the chronic condition, the agency said in its statement Friday that Botox had not been shown to work for the occasional headache or migraine.
Common side effects were neck pain and headaches. But neurologists point to a more welcome side effect for some — fewer wrinkles.