"My patients love, love, love AHAs," says Downie, who explains that the powerful exfoliators are genius for clearing up sun damage, hyperpigmentation, acne, and fine lines. Multiple AHAs exist, but the most popular (and potent) is glycolic acid, which penetrates damaged skin to spur fresh, new skin cell production. Glycolic acid does its exfoliating work in everything from once-monthly in-office face peels to nightly washes, but it's best not to use glycolic acid while you're on retinols. And if your skin is sensitive, try glycolic's less intense AHA cousin, lactic acid, which also chemically exfoliates but isn't as drying.” – Glamour