Is this strictly correct? I thought the problem with thalidomide was that both enantiomers could be metabolised, but in different pathways with vastly different effects (ie. one reduced morning sickness, the other was a teratogen without reducing nausea).

Why a cosmetic product would be bragging about a “chiral” ingredient makes no sense. Chirality in and of itself tells you nothing about something’s properties. It would be a little more meaningful if they told you which of the two enantiomers was an ingredient, but even then, it seems irrelevant. Opposite enantiomers have exactly the same properties as one another, with only two exceptions: they rotate polarized light in opposite directions (which has nothing to do with cosmetics, or anything else, for that matter), and sometimes the body can only metabolize one enantiomer and not the other. Almost all the sugars your body can metabolize are D-sugars, whereas all the amino acids your body can metabolize are L-amino acids. D and L are the two types of enantiomers. A sugar packet could say “contains a chiral ingredient”–but since all sugars are chiral, this is redundant. What’s more, saying the sugar in the packet is chiral doesn’t even mean it’s the correct enantiomer. It could be the worthless L-sugar that would go right through you. But none of this matters, since the original post says that there are no enantiomerically specific metabolic pathways in skin absorption.

It’s nice to know those tuition checks have been well spent.