Patients who started out having headaches almost daily reported two fewer headaches per month when they were given injections of botulinum toxin A. They also had more side effects, including weak muscles and a stiff neck.
The medication is marketed under multiple brands, but Allergan's Botox is the best known. Botox is used to treat a range of conditions, including migraines and excessive sweating.
"The effect these appear to be having on migraine headaches is small -- it only reduces headaches by a couple of days a month," said Dr. Jeffrey Jackson, the study's lead researcher, from the Medical College of Wisconsin in Milwaukee.
That's "really, really modest," he told Reuters Health.
Still, it's possible that some chronic migraine patients will benefit from injections much more than others, he said.
For their new analysis, he and his colleagues looked back at 27 studies in which more than 5,000 headache patients were randomly assigned to get botulinum toxin A injections in the head and neck or an injection of a drug-free placebo. The study was published on Tuesday in the Journal of the American Medical Association.
The initial trials varied widely in their use of the drug, with researchers injecting the medication into any of four to 58 spots, either at a single time or at three different times a few months apart.
Most trials allowed patients to use other headache medication in addition to the injections.
Study participants with chronic headaches or chronic migraines initially reported having 17 to 20 headaches per month, on average. Twelve weeks or more after getting botulinum toxin A injections, that had dropped by an average of two monthly headaches, compared to patients getting the placebo.
In people with fewer migraines to begin with -- six per month, on average -- the drug injections didn't seem to provide any change in headache frequency. The findings were consistent regardless of patients' age as well as the botulinum toxin A dose and injection strategy used.