Retinoic acid, or vitamin A, may protect skin from sun damage that leads to premature aging and skin cancer. Retinoic acid is available by prescription under the brand name of Retin A, and is used to treat acne and to reduce skin wrinkling that has already occurred. In their study, the researchers determined that ultraviolet radiation effectively causes a functional vitamin A deficiency in skin by reducing the expression of certain proteins necessary to maintain levels of vitamin A. Vitamin A is essential for normal skin development, but the sun's ultraviolet (UV) radiation can cause a vitamin A deficiency even before sunburn is noticeable, the investigators report. Taking vitamin A orally would have no effect on the receptor-loss caused by UV radiation. When you apply retinoic acid topically however, it saturates those receptors and that's what you need.
Nature Medicine April 1999;5:376-377, 418-422.
COMMENT: This is one of the few drugs which I believe to be highly beneficial. I have used it myself for over two decades. It works especially well with alpha hydroxy acids. One only needs to be cautious and not apply it to your face if there is any redness on it. It is a prescription item though. And since finding it works well to get rid of wrinkles, the price nearly doubled. So it is quite expensive to use unless you have a prescription card or are a physician getting samples. Highly recommended.