An injectable formulation of a neurotoxin derived through the fermentation of the Hall strain of Clostridium botulinum type A with neuromuscular transmission inhibitory and analgesic activities. Upon injection into the affected muscle, the heavy chain portion of onabotulinumtoxinA binds to the cell membrane of the motor nerve and is internalized via endocytosis. Upon entry, the light chain portion of the toxin is activated and cleaves the protein SNAP-25, thereby preventing the fusion of acetylcholine (ACh)-containing synaptic vesicles with the cell membrane and, so, the release of ACh into the neuromuscular junction; subsequent binding of ACH to motor end-plate nicotinic acid receptors and ACh-mediated muscle contraction are thus blocked. In addition to ACh, onabotulinumtoxinA may inhibit the release of neuropeptides, such as substance P and glutamate, which may contribute to its analgesic activity. OR A toxin made by the bacterium Clostridium botulinum. It can cause food poisoning. The drug Botox is a form of the toxin that can be used in small amounts to treat certain medical conditions. These include severe underarm sweating and severe muscle spasms in the neck and shoulders. Botox is also used to smooth wrinkles on the face. It is being studied in the treatment of pain in patients with skin leiomyomas (benign smooth muscle tumors) and other conditions. Also called botulinum toxin A and botulinum toxin type A.