There are a multitude of factors that lead to underarm discoloration, but the most common cause is skin friction, according to the board-certified dermatologists we talked to. “Constant friction between the skin or with tight clothing can cause hyperpigmentation due to the irritation it causes,” says Mina Amin, MD, a board-certified dermatologist based in Los Angeles. For our list of the best armpit brightening creams, we considered each product’s performance across five primary categories: product ingredients and efficacy, packaging, fragrance, texture, and product wear.