According to Zeichner, most over-the-counter retinol products don't contain more than 1% retinol, though some products have up to 2% or higher. It's also important to keep in mind that a retinol concentration below 0.25% won't be effective. "Companies aren't required to disclose the concentration of retinol in products, and many keep this information private as part of the trade secret of the formula," he explains. That said, if a brand doesn't list the concentration, you may want to confirm with them directly to ensure the product is effective.