Botulinum toxin type A has been used to inhibit synaptic exocytosis, reducing muscle tone. Several studies have demonstrated that this toxin may have analgesic activity, independent of its effect on muscle tone. This study assessed the efficacy of botulinum toxin A for the treatment of peripheral neuropathic pain.

This randomized, double-blind, placebo controlled, parallel group, clinical trial included patients with neuropathic pain scores of at least four out of 10, with daily pain for at least six months, attributable to a peripheral nerve lesion. One week after baseline assessment, patients were randomized to receive two, subcutaneous administrations of botulinum toxin A up to 300 units or an equal volume of placebo, 12 weeks apart. The primary outcome variable was the change in self-reported pain intensity after two successive injections, over the prior 24 hours, using the 11-point Brief Pain Inventory (BPI). All secondary endpoints were assessed at four, 12 and 24 weeks, including safety and tolerability and therapeutic gain.

Of the 66 adults included in the intent to treat analysis, the mean pain intensity was 6.5 at baseline and 4.6 at week 24 in the botulinum toxin group, and 6.4 at baseline and 5.8 at week 24 in the placebo group (P<0.0001). The difference in reduction in pain intensity between the groups was significant after the first administration, starting from week one, and increased between weeks 15 and 24. The proportion of those who responded with at least a 30% reduction in pain at week 24 was greater in the treatment group (P＝0.001). This was not true of the proportion who responded with at least a 50% reduction in pain (P＝0.2)．

This study found that two administrations of botulinum toxin A resulted in a sustained analgesic effect against peripheral neuropathic pain.