Large group of over 2,500 chemicals related to vitamin A. Topical, over-the-counter retinoids include retinol, retinyl palmitate, retinaldehyde, and retinyl linoleate, among others. Prescription retinoids include tretinoin (Renova, Retin-A), adapalene (Differin), and tazarotene (Tazorac). Other retinoid chemicals include beta-carotene and various carotenoids found in brightly colored and dark green fruits and vegetables.
When applied topically, retinoids function in multiple ways. Primarily, they work as cell-communicating ingredients, essentially connecting with a receptor site on a skin cell and “telling” it to behave in a more normal, healthier manner. Retinoids have benefits for more than 125 different skin issues, from acne to psoriasis to wrinkles and other signs of sun damage. They can improve, to some extent, how new skin cells are formed and how they behave as they mature and make their way to the surface of your skin.
Which retinoid to use is a matter of personal need and experimentation; generally, retinol and other over-the-counter forms are used for wrinkles, firming skin, and sun damage. Prescription retinoids such as tretinoin are used for signs of aging as well as acne, while adapalene is used most often for acne. Tazarotene is often used when acne and wrinkles are present, but all retinoids have value for both concerns. Over-the-counter retinol can be helpful for acne, too, but for stubborn breakouts the prescription retinoids tend to work better (albeit with a stronger potential for side effects).
Tolerability can be an issue with all retinoids, though some (such as adapalene) are less irritating than others. The most common side effects from topical application of retinoids include irritation, flaking skin, and redness (sometimes resembling and/or feeling like sunburn, with skin being tender to the touch). Side effects tend to appear within 2–4 days from the time you applied the retinoid. In most cases, they subside within a few weeks as skin adapts; however, there will always be some people whose skin is simply intolerant of retinoids. It is important to avoid applying too much of any retinoid product; more isn’t better and can often make the potential side effects an unwelcome reality. For example, the directions for prescription retinoid creams state to apply a pea-sized amount, which is plenty. Applying more per usage won't get you better or faster results, but can increase the chance for unwanted side effects.
There is research showing that varying strengths of retinol and prescription retinoids are beneficial; with any type of retinoid, the "if a little is good, more must be better" mentality can backfire. Some people's skin can tolerate higher amounts of retinoids, but it's always best to begin with the lower strength to see how skin responds and then increase the concentration from there if results are positive. It is also fine to alternate between lower- and higher-strength retinoid products; for example, one night you can apply an over-the-counter retinol product and the next evening apply a prescription retinoid.
Sources: Collegium Anthropologicum, September 2010, pages 1,145–1,153; Clinical and Experimental Dermatology, August 2010, Epublication; British Journal of Dermatology, July 2010, Epbulication; Journal of Drugs in Dermatology, October 2009, pages 932–936; Cosmetic Dermatology, Second Edition, Baumann, Leslie M.D., 2009, pages 256–259; and Clinical Interventions in Aging, Volume 1, Issue 4, 2006, pages 327–348.