The health watchdog NICE has said it wants more information before it recommends an anti-wrinkle jab as a form of NHS treatment for chronic migraine sufferers.
The watchdog published draft guidelines which said there is little proof so far that Botox can be used as a treatment.
Allergan, which makes Botox, has been told it must provide more information before it can make a final decision.
Some 1.6 per cent of adults are thought to be affected by chronic migraines.
The watchdog said that if eight out of 15 headaches a person has in a three-month period are migraines, they are a chronic migraine sufferer.
It is thought that the muscle relaxant Botox - or botulinum toxin - may block pain signals, but the exact reasons that it may work for migraines are unknown.
The jab was approved as a UK treatment for the prevention of chronic migraine in July 2010, by the Medicines and Healthcare Products Regulatory Agency.
NICE is considering whether the NHS in Wales and England should offer the injection.
After evidence from Allergan was reviewed, the independent committee at NICE discovered that Botox has “some benefit” in clinical trials.
However, the benefits were small, NICE said, and the findings were “confounded by a large placebo effect”.
It is expected a final decision will be made in June.