Botulinum toxin is the most poisonous substance known. A single gram of crystalline toxin, evenly dispersed and inhaled, would kill more than 1 million people, although technical factors would make such dissemination difficult. The basis of the phenomenal potency of botulinum toxin is enzymatic the toxin is a zinc proteinase that cleaves 1 or more of the fusion proteins by which neuronal vesicles release acetylcholine into the neuromuscular junction. It has become the first biological toxin to become licensed for the treatment of human disease. Botulinum toxin is currently licensed for the treatment of cervical torticollis, strabismus, and blepharospasm associated with dystonia.