The Food and Drug Administration on Friday approved Botox, the anti-wrinkle shot from Allergan, as a treatment to prevent chronic migraines, a little more than a month after the company agreed to pay $600 million to settle allegations that it had illegally marketed the drug for unapproved uses like headaches for years.

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 approved Botox 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.