Adapalene
In plain terms
A modern retinoid for acne, and the first one sold over the counter (as Differin). It works well and is more stable in light than older retinoids, but it dries and irritates skin while it adjusts, and it is held off during pregnancy as a precaution.
What the evidence shows
Adapalene is a third-generation retinoid, designed to lock onto the skin's retinoid receptors more selectively than tretinoin, which is part of why it tends to irritate a little less while still clearing acne. It cleared the bar for over-the-counter sale in 2016, the first retinoid to do so. On the skin its safety story is mostly about tolerance: dryness, redness, peeling, and stinging are common early on and usually ease with time and a slower pace. Two things people worry about hold up less than expected. It is not a proven sun-sensitizer, and is in fact more photostable than tretinoin, though sun protection is still sensible. And the cancer and gene-damage tests on topical adapalene have come back clean. Pregnancy is the real caution, and it deserves care. Swallowed retinoids cause birth defects, and high oral doses of adapalene are teratogenic in animals, but applied to skin less than 0.01% gets into the body, and two decades of prescription use turned up no clear pattern of retinoid birth defects. Even so, regulators in the US and EU say to avoid it in pregnancy or when planning one, a precaution rather than a demonstrated topical harm. So it is a strong acne option for most people, with irritation to manage and pregnancy as the line to respect.
On the label
Listed as adapalene, usually at 0.1% over the counter and 0.3% by prescription. The packaging carries the pregnancy caution for a reason; introduce it two or three nights a week at first, pair it with moisturizer, and skip it if you are pregnant or trying.