This document summarizes the clinical pharmacology of botulinum toxin A. It discusses how botulinum toxin works by inhibiting the release of acetylcholine at neuromuscular junctions, its mechanism of action on cleaving proteins involved in vesicle fusion, and its temporary effects that require repeated injections. The document also outlines botulinum toxin's introduction as a treatment for muscle overactivity and facial aesthetics, its time course of onset and duration of effects, dosing in units, and potential adverse events being primarily localized muscle weakness.