Tretinoin.

"Topical retinoids are vitamin A derivatives. They work in the epidermis as well as in the dermis. They don't dissolve bonds between stuck together skin cells, but rather they actually bind in the cell to specific receptors to influence cellular behavior in the epidermis. This helps to normalize the rate of maturation of the epidermis and epidermal turnover in the deeper layers of the skin. This helps to stimulate new healthy collagen production over time. This is key for retinoids impacts on improving fine lines, wrinkles, and skin firmness with ongoing use. Topical retinoids also help with pigmentation by not only hastening the clearance of cells that are laden with unnecessary amounts of pigment in the epidermis, but also by having some impacts at the level of the pigment producing cell, the melanocyte, in terms of reducing unnecessary pigment production. Tretinoin is available in the United States by prescription only. And it's also available through many direct to consumer websites."