But did you know that we can treat this condition with a few injections of Botox? When used to treat underarm excessive sweating, 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 12 months but some studies have found it can last as long as 14 months.