By Corrire Pikul

Everyone's got their favorite tricks for younger, more radiant skin. But this ingredient tops the must-try lists of all the experts and has reams of scientific support.

Ask a dermatologist for her No. 1 skin-care recommendation and you know you'll hear something along the lines of: "Wear SPF! Seriously! What's it going to take to get you people to put it on?!" So we asked our experts for their second most important piece of advice, and every one of them said the same thing: "Use retinoids." These vitamin A derivatives have been proven to boost collagen production (reducing wrinkles and preventing fine lines), speed cell turnover (evening out discoloration and brightening skin tone) and unclog pores (making them look smaller and reducing acne). "I can always tell when a woman uses these. She'll have that dewy, fresh-faced look—like she just had a facial," says Emmy Graber, MD, a dermatologist and assistant professor of dermatology at the Boston University School of Medicine. You'll want to know these 4 things before giving retinoids a try.

You don't need an Rx, but here's why you might want one.



Tretinoin, the most common prescription retinoid, is stronger, more potent and faster-acting than retinol, the OTC version, says Graber. In fact, the lowest Rx concentration can be 20 times stronger than the highest amount allowed by the FDA in drugstore products, one study showed. But because retinoids have been known to cause redness and dryness among the newly initiated, the OTC products can be an excellent starting point for sensitive skin. Graber says that the lower-concentration retinoid products may be especially beneficial for the eye area, where the skin is thinner and more delicate than the rest of the face.

They have something (very important!) in common with Gremlins.



No direct sunlight—not when using any kind of retinoid. Not only is retinoid-treated skin super-sensitive to UV rays and likely to burn, but the sunlight also makes the product less effective. That's why dermatologists strongly recommend putting these products on your face before bed and using a sunscreen every morning. ("If I have a patient going on a beach vacation, I'll even tell her to stop using Retin-A for a week prior to departure," Graber says.)

Once you start, you won't want to go back.



"I don't know anyone over age 25 who could not benefit from a nightly retinol product," says Brooke Jackson, MD, a dermatologist and associate professor of dermatology at UNC at Chapel Hill. You may notice results in as little as four weeks if you're using a stronger prescription product; opt for an OTC cream, and it will take closer to 12 weeks for you to notice a difference. The benefits last for as long as you keep up the routine, as skin doesn't stop responding, and retinoids haven't been proven unsafe in prolonged use. (Just be aware that even long-term users may need to take breaks, as retinoids are not considered safe for women who are pregnant or breast-feeding.)

This is the most costly mistake people make with retinoids.



They put on too much (because they think more is better) but you only need a pea-size amount for your entire face, Graber says. Slathering on retinoid creams won't only cause you to quickly use up your supply, but it can also cause redness, dryness and irritation. Even when used judiciously, retinoids can be quite drying. That's why Graber recommends a moisturizer just before bed.