Retinol
In plain terms
The over-the-counter vitamin A that skin converts into its active anti-aging form. It works, and the same activity that makes it work is why it can irritate and why pregnancy is the real line to watch.
What the evidence shows
Retinol is vitamin A in a form your skin converts into retinoic acid, the molecule that drives the anti-aging and acne benefits, and that conversion is also where its two real considerations come from. The first is irritation: retinol reliably causes redness, dryness, and peeling at higher strengths, which is dose-dependent and why lower concentrations introduced slowly are better tolerated. The second, and the one that matters most, is pregnancy. Vitamin A taken into the body in excess is a known cause of birth defects, and while topical retinol absorbs only slightly, Europe's safety committee judged that cosmetics, food, and supplements together could push total vitamin A past the safe ceiling. Europe's response was telling: not a ban, but a cap at 0.3% for most leave-on products and a label reminding people to mind their total intake, with the committee calling it safe at those limits. So the honest read is that retinol is effective and fine for most non-pregnant adults at sensible strengths, worth easing into for irritation, and worth setting aside entirely during pregnancy. One thing it is not: the skin-cancer worry sometimes attached to vitamin A belongs to a specific question about retinyl palmitate under UV, not to retinol at night, which is how it is normally used.
On the label
Listed as retinol or sometimes just vitamin A. If you are pregnant or trying to be, this is the category to set aside; otherwise lower strengths and slow introduction handle most of the irritation.