Botox is of little use as a treatment for chronic migraine despite being licensed in some countries for the condition, experts say.
The anti-wrinkle jab was approved in the UK in July last year for treating chronic migraines, which are thought to affect about 700,000 people there.
A trial of more than 1,300 patients showed success in reducing the frequency of headaches.
People in Britain 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.
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, with a similar proportion developing itching, rash, pain, stiffness and muscle spasms, they add.
Rarely, Botox can prompt anaphylactic shock.