Benzoyl peroxide is an effective antibacterial ingredient. It kills all bacteria on the skin. It is commonly used to treat acne on the face. It is considered a short term solution. In the underarms, it is normal that there are many bacteria, given the warm, moist, and nutritiously rich conditions. If I were a bacterium, I would love living in the underarm! So bacteria will always come back, no matter what is applied. Best, Chris