There may be thousands of products swearing to zap your zits, but there's one ingredient at the heart of many that's actually doing the work: salicylic acid. Your first encounter with the ingredient was probably around age 13; now it's a mainstay in your medicine cabinet. But what does it actually do? Do you need 0.5% or 2%? How often can you use it? We checked with an expert, dermatologist Dr. Murad, to get the answers and picked out a few of our favorite salicylic acid-based formulas, below.