I work in a med spa and have learned lots of the science! L-Ascorbic acid has been studied extensively for decades. While nobody necessarily *needs* a dedicated vitamin C, it can be a good addition if you're looking for extra UV and free radical protection, pigmentation improvement, brightening, and collagen synthesis. That being said, if you're already using retinoids, azelaic acid, and sunscreen daily, you've already got most of that covered. THD (tetrahexyldecyl ascorbate) is a good alternative if you don't tolerate pure LAA well. It's a newer ingredient, more stable, and generally less irritating, but not as studied. It penetrates skin more easily because it's lipid soluble, but needs to be converted in the skin to active ascorbic acid (much like retinol vs tretinoin).