A double-blind, randomized, placebo-controlled study of patients with essential trigeminal neuralgia and treatment with a single injection of onabotulinum toxin A (BTX) was carried out. The efficacy, safety, and tolerability of either 1 mL 0.9% saline plus 50 U of BTX or only 1 mL of 0.9% saline injected subcutaneously in the affected area were evaluated. Cases with involvement of the third branch of the trigeminal nerve also received intramuscularly either 10 U of BTX or matching placebo in the masseter muscle, ipsilateral to the pain location. Pain was assessed with the visual analog scale (VAS). Twenty subjects were administered BTX, and 16 subjects received placebo. Two months after the intervention, a trend to statistical significance was observed for the VAS mean values in subjects treated with BTX and those who received placebo (VAS 4.9 vs 6.63, t test, P = 0.07). Three months after the injection, significant differences were observed in the average VAS score for subjects treated with BTX and those treated with placebo (VAS 4.75 vs 6.94, respectively; t test, P = 0.01). Onabotulinum toxin A was well tolerated and seems to be a safe and useful therapy for patients with essential trigeminal neuralgia.