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The two are often believed to be interchangeable, but the main difference is that retinol is eventually converted into retinoic acid. Retinol is a natural form of vitamin A, while Retin-A is derived from retinoic acid, a synthetic derivative of vitamin A, and considered pharmaceutical.
Retinol is a weaker, over the counter version of tretinoin, a prescription vitamin A derivative that is highly effective in reversing sun damage and signs of aging. Tretinoin decreases fine lines, evens skin color, improves texture, tightens pores, and stimulates blood flow and collagen production.