Normally, the nervous system activates sweat glands by releasing acetylcholine when body temperature rises. In hyperhidrosis, the nerves signalling sweat glands are overactive, producing excessive sweat unrelated to heat or exercise. Botox injected into the underarm blocks acetylcholine release at the nerve-sweat gland junction, preventing sweat gland activation. Within 2 to 4 weeks, sweating reduces dramatically and remains suppressed for 6 to 12 months until the nerve endings regenerate.