Botox is of little use as a treatment for chronic migraine despite being licensed for the condition, experts say.
The popular anti-wrinkle jab was approved in July last year for treating chronic migraines, which are thought to affect about 700,000 people in the UK.
A trial of more than 1,300 patients showed success in reducing the frequency of headaches.
People can be treated with Botox, including on the NHS at their doctor's discretion, if they suffer headaches on at least 15 days per month of which at least eight days are with migraine.
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. But experts from the Drug And Therapeutics Bulletin said there was "limited evidence" of Botox working for this group of patients.
They questioned the selection of patients used in the clinical trial for the drug, saying the diagnosis of chronic migraine was incorrect as almost two thirds of trial participants overused headache treatments.
Botox leads to worsening of headache symptoms in around one in 10 people, with a similar proportion developing itching, rash, pain, stiffness and muscle spasms, they add.
Rarely, Botox can prompt anaphylactic shock and the possibility of transmitting an infection with Botox cannot be ruled out.
"These discrepancies and the limited evidence of benefit make it difficult for us to see a place for botulinum toxin A as treatment for chronic migraine," the experts said.
The recommendation is for Botox to be given at between 31 and 39 different sites on the head and neck, with re-treatment every 12 weeks. The cost is around £276 per treatment.