Prodrug

Two of the most useful molecules in skincare are awkward to put in a jar. Ascorbic acid oxidises readily and wants an acidity not everyone tolerates. Retinoic acid is a prescription medicine in many markets and is not a gentle one. The workaround in both cases is the same: supply something adjacent — an ester, a salt, a sugar conjugate — and let the skin take the last step.

Three things have to be true for that to pay off. The derivative has to reach the skin. Something in the skin has to cut it. And enough of the result has to survive long enough to matter. Each is a place to lose material, and the losses multiply rather than add: clear all three at a comfortable-sounding majority and what arrives is still a fraction.

So a percentage on a derivative is not a percentage of the active, and ten of something that converts poorly can deliver less than one of the parent. That is not an argument against derivatives. A stable, comfortable product someone actually keeps using is worth more than a potent one that browns in a fortnight.

It is an argument for reading the claim closely. “Converts to X in the skin” describes a mechanism. “Delivers as much X as Y” describes conversion efficiency, and that is a number measured far less often than it is asserted — most often by whoever sells the raw material.