The anti-wrinkle drug Botox, which has been licensed in some countries as a treatment for chronic migraine, has negligible effect against these headaches, a study published on Tuesday said.
Botox - a neurotoxin derived from the Clostridium botulinum germ - works as a cosmetic by paralysing the muscles that cause wrinkles.
It has now been authorised in Britain, the United States and elsewhere to relieve chronic migraines, administered by regular injections to up to 39 sites in the head and neck muscles.
But the British journal Drug and Therapeutics Bulletin said clinical trials showed only "limited evidence of benefit."
Between one and 10 percent of volunteers who were injected with the drug experienced uncomfortable or painful side effects, including muscle spasms, rashes and itches.
The report added it was impossible to completely rule out other risks from Botox, including the risk of infection from a protein called human serum albumin, which is part of the drug's formula.