A health watchdog has called for more information before it can recommend Botox for the treatment of chronic migraines on the NHS.
Draft guidelines published by the National Institute for Health and Clinical Excellence (Nice) say there is insufficient evidence so far to back the anti-wrinkle jab as a treatment option.
It wants the manufacturer, Allergan, to provide more data before a final decision can be made.
Chronic migraines - believed to affect 1.6% of adults - involve having headaches for at least 15 days a month over three months, eight of which are migraines, the watchdog said.
It is not known exactly why Botox, or botulinum toxin, may work for migraine although it is thought it may block pain signals as well as being a muscle relaxant.
In July 2010, the Medicines and Healthcare Products Regulatory Agency approved Botox as a preventive treatment for chronic migraine in the UK.
Nice is considering whether the injection should be offered across the NHS in England and Wales.
After reviewing evidence provided by Allergan, Nice's independent committee found that Botox was shown to have "some benefit" in clinical trials.
But it said the amount of benefit was small and the results were "confounded by a large placebo effect".
The committee also said the long-term effectiveness of the drug is uncertain.
Nice estimated the cost to the NHS would be £349.40 for every 12-week cycle of treatment.
Professor Carole Longson, director of the health technology evaluation centre at Nice, said: "Our independent committee is asking Allergan to provide further information and analysis as part of this public consultation, so that it has sufficient evidence to develop sound advice for the NHS regarding the use of Botox for the prevention of headaches in adults with chronic migraine.
"Without this additional evidence, potentially we will be unable to advise the NHS that this drug is good value for money for these adults because there are currently too many uncertainties."
A final decision is expected in June.