Botox treatment helps control the symptoms of severe underarm sweating by temporarily blocking the chemical signals from the nerves that stimulate the sweat glands. When the sweat glands don't receive chemical signals, the severe sweating stops.
Botox injections are expected to temporarily stop the production of excessive sweat in the treated areas only. Sweat continues to be produced elsewhere.
Where does the sweat go?
Your sweat doesn't go anywhere or get backed up because the sweat simply is not produced in the areas treated with Botox. Remember, the rest of your body is free to produce moisture normally.