Botox can be used to treat underarm excessive sweating, also known as hyperhidrosis. When used to treat hyperhidrosis, Botox has been shown to result in an 82-87% decrease in sweating. Results start to be noticeable approximately 2 to 4 days after treatment with the full effects usually noted within 2 weeks. Dryness typically lasts 4 to 6 months.