Botulinum toxin may reduce the number of migraine days for people with chronic migraine by two per month, the first Cochrane review of the treatment has found.

In a systematic review of 23 trials of botulinum toxin type A for chronic migraine (defined as more than 15 migraine days per month), the treatment was found to reduce migraine days by three days more than placebo, but the benefit was reduced only two days when small trials were excluded.

“We also saw a reduction of 30% more in the severity of migraines in the treated group than in the placebo group but this was based on very low‐quality evidence,” said the authors of the review, from the Department of Neurology, at Birmingham University, UK.

The reviewers found the overall quality of evidence for botulinum toxin in migraine prevention to be low, with just two larger trials providing robust evidence. Most of the trials were also short term, lasting just a few weeks or months so there was a lack of long term data on efficacy and safety of botulinum toxin.

The review also noted there was inadequate evidence to support the use of botulinum toxin in episodic migraine (less than 15 migraines per month).

“We did not identify sufficient data to assess whether botulinum toxin treatment should cease on reduction of headache days below the current 15‐day chronic migraine cut off,” they said.

“Our conclusions are in line with current clinical guidelines. We could not draw conclusions about the order in which botulinum toxin and other prophylactic agents should be used to treat migraine as there was insufficient evidence on comparative efficacy and safety,” the report authors concluded in their review’s implications for clinicians.

Adverse effects were seen in 60 out of 100 patients treated with botulinum toxin, the most common being drooping eyelid or muscle weakness.

Botox was listed on the PBS for treatment of chronic migraine in March 2014, and a recent 2017 Drug Utilisation Sub-Committee (DUSC) review found that the rates and duration of use by neurologists had been “substantially higher than predicted”.

There were 3,517 and 5,444 patients treated with botulinum toxin for chronic migraine in the first and second year of PBS listing, respectively. The continuation rate on botulinum toxin treatment at 24 weeks (i.e. after two treatments) was 71.4%, which the DUSC noted was more than double the 33% predicted from trial data.