Ingredient index

Retinol

The most studied over-the-counter retinoid; two conversion steps from retinoic acid.

What it can share a night with

With	Verdict	Why
Salicylic acid	Alternate nights	Both increase turnover, and stacking them is the most common route to the stinging and flaking people mistake for sensitive skin. Alternate nights and the irritation usually resolves without dropping either.
Vitamin C	Use with care	Commonly repeated advice that they cannot be layered, which is weaker than it sounds.