Retinoic acid (tretinoin, vitamin A acid, ATRA) is included in skin treatments for acne and is also available as a nutritional supplement. All retin-A products are biochemical exfoliators that remove the outermost layer of skin cells and encourage new ones to form. This is particularly helpful for acne as it is the outermost layer of skin that contains acne-causing bacteria. Retinoic acid was the first retinoid developed for topical use, so in addition to acne treatments, it commonly appears in anti-aging formulas, as well as a products claiming to improve the appearance of stretch marks. In these applications, it’s the double benefit of exposing younger skin, as well as increasing new collagen to develop. It’s being studied for prevention and treatment of certain types of cancer. The one big problem with formulas that contain retinoic acid is that dryness, redness, scaling, itching, and burning can occur. Doctors often recommend introducing it in stages of frequency and concentration in order to build up tolerance. As with all acid-based ingredients, sensitivity to the sun’s rays is increased. Shade, sunscreen, and coverups are recommended.