SANTA ANA, Calif. – The Food and Drug Administration has approved Botox as a treatment for chronic migraines.

Irvine, Calif.-based Allergan, which makes Botox, announced Friday that it had received a letter from the FDA granting the approval for the use of Botox as a method of preventing migraines in patients who have the headaches an average of 15 or more days per month and lasting four hours a day or more.

An estimated 3.2 million Americans are affected by chronic migraines, the company said.

In a typical treatment session, a headache specialist such as a neurologist injects a total of 155 units of Botox into seven sites in the head and neck, an Allergan spokeswoman said.

For doctors, the price for Botox will remain $525 for a 100-unit vial, according to the Wall Street Journal.

In many cosmetic Botox sessions, doctors inject a total of 100 units of the diluted form of botulinum toxin into the forehead and around the eyes. But the FDA has only approved cosmetic injections of 20 units to treat wrinkles between the eyebrows.

The safety and effectiveness of Botox has not been established for the treatment of episodic migraine, Allergan said. In July, British regulators approved Botox for migraine sufferers.

Allergan has been running clinical trials of Botox as a treatment for migraines since 2004.

In one trial, patients treated with Botox had 7.8 fewer days per month of headaches, compared to 6.4 fewer days with a placebo, BusinessWeek reported. In another trial, patients getting Botox had 9 fewer days of headaches, compared with 6.7 days fewer for an inactive injection. In the United States, because Botox is approved for other uses, doctors such as Dr. Susan Hutchinson at the Orange County Migraine and Headache Center in Irvine have already prescribed it routinely as a treatment for migraine sufferers. Observers predict that FDA approval will expand the number of doctors willing to offer Botox as a migraine treatment.

Botox sales total $1.3 billion a year, and analysts have said that FDA approval of Botox as a migraine treatment could add $1 billion to that total. In a news release, the FDA said the Botox injections will typically be timed every three months. In previous tests, the most common side effects for migraine patients treated with Botox were neck pain and headache.

The FDA noted the potential risks of Botox:

“OnabotulinumtoxinA, marketed as Botox and Botox Cosmetic, has a boxed warning that says the effects of the botulinum toxin may spread from the area of injection to other areas of the body, causing symptoms similar to those of botulism. Those symptoms include swallowing and breathing difficulties that can be life-threatening.

“There has not been a confirmed serious case of spread of toxin effect when Botox has been used at the recommended dose to treat chronic migraine, severe underarm sweating, blepharospasm or strabismus, or when Botox Cosmetic has been used at the recommended dose to improve frown lines.”