RetinoidsRetinol · Tretinoin
Vitamin-A derivatives: the best-evidenced topical active for skin ageing.
Retinoids are vitamin A derivatives used on the skin, ranging from over the counter retinol and retinaldehyde to prescription tretinoin and, for acne, isotretinoin taken by mouth. They are the most thoroughly evidenced topical actives in dermatology.
They work by binding receptors in skin cells and changing which genes are expressed, which speeds cell turnover, normalises the way skin cells shed into pores, and over months increases collagen production in the dermis. That is why they help both acne and photoageing, two problems that otherwise share little.
Potency runs in a chain. The skin converts retinol to retinaldehyde and then to retinoic acid, the active form; prescription tretinoin is retinoic acid already, so it works faster and irritates more. Each conversion step loses some potency, which is the honest reason an over the counter product is gentler and slower.
Expect months rather than weeks. Meaningful change in fine lines and pigmentation takes something like three to six months of consistent use, and the first few weeks often look worse rather than better as the skin adjusts, which is when most people give up.
Irritation is the main obstacle and it is largely manageable: start two or three nights a week, apply to dry skin, use a bland moisturiser, and build up. Sun protection is not optional alongside them, both because the skin is more sensitive and because there is no point rebuilding what the sun is degrading.
Oral isotretinoin is a different proposition entirely. It is highly effective for severe acne, it is prescription only, it requires monitoring, and it causes serious birth defects, so pregnancy prevention is a formal part of treatment.
In practice, a retinoid is the one active with enough evidence to justify a place in most routines, and consistency over months matters far more than which brand or concentration you choose.