How retinoids work

Retinol, retinal, adapalene, tretinoin — what's the difference?

Short answer: they're not rival products. They're different strengths of the same thing, all headed for the same place inside your skin cells.

Retinoids are the most-studied ingredient in dermatology, and the most misunderstood. Retinol, retinaldehyde, adapalene, tretinoin — they aren't competing products so much as different doses arriving at the same destination. Understanding that destination explains why they work, why they irritate, and why they take months.

From cream to gene

A retinoid that isn't already retinoic acid gets converted, step by step, inside your skin cells: retinyl esters turn into retinol, retinol into retinaldehyde, retinaldehyde into retinoic acid [8]. Retinoic acid then acts like a hormone — it goes into the cell's nucleus and binds two families of receptors, the retinoic acid receptors (RAR) and retinoid X receptors (RXR), which sit on the DNA and switch sets of genes up or down [3]. That's the whole trick: retinoids don't scrub anything off mechanically; they change which genes are active.

What those genes do

Two results matter most for skin. First, retinoids get keratinocytes — the cells lining the pore — maturing and turning over normally, which is why they unclog pores and head off the tiny early clogs (microcomedones) that grow into acne [7]. Second, they tilt the collagen balance. UV light ramps up enzymes called MMPs that break collagen down — a major driver of photoaging — and pretreating skin with tretinoin was shown to block that induction by 70–80% [6]; retinoids also support new collagen directly — a controlled biopsy study measured an 80% rise in type-I collagen versus a 14% drop with the vehicle cream [5]. The classic 1988 trial had already shown prescription tretinoin significantly improved sun-damaged skin versus a dummy cream [1]; the biopsy work confirmed the change is real repair, not just surface smoothing.

The strength ladder, and the irritation that rides with it

Because the over-the-counter forms have to convert, the number of steps predicts strength: retinol is two steps from retinoic acid, retinaldehyde one, and retinyl esters are the gentlest and weakest [8]. Adapalene is different — it's a lab-made retinoid that binds the receptors directly and selectively, which is why it's effective for acne, relatively well tolerated, and sold over the counter [4]. More strength generally means more of the early irritation ("retinization") [8] — dryness, flaking, pinkness that peaks in the first weeks and then settles. The practical how-to-start — go low and slow, buffer with moisturizer — is in starting retinoids.

Why sunscreen isn't optional

Retinoids make skin more sun-sensitive, and some (tretinoin especially) break down in UV light — which is why they're night-time products and why daily sunscreen is part of the deal [2]. Using a retinoid without sunscreen works against the very collagen you're trying to build. And one caution worth flagging: the vitamin-A pathway is the one with a proven risk in pregnancy (oral isotretinoin is known to harm a pregnancy), so topical retinoids are generally paused as a precaution — our pregnancy check covers which actives to swap, and a clinician should confirm.

If retinoids consistently overwhelm your skin, bakuchiol matched 0.5% retinol in a head-to-head trial with less irritation [9], and is a reasonable alternative to consider. The deeper dive on results and evidence is in retinoids and photoaging.

FAQ

What's the difference between retinol, retinal, adapalene, and tretinoin?

They're all retinoids that act on the same receptors, but they get there differently. Retinol and retinaldehyde (retinal) are over-the-counter forms your skin converts toward retinoic acid — retinal is one step away, retinol two, so retinal tends to be a bit stronger. Adapalene is a lab-made retinoid that binds the receptors directly and is sold over the counter for acne. Tretinoin is retinoic acid itself, a prescription drug with the deepest evidence. Fewer conversion steps generally means more strength and more potential irritation.

Why do retinoids take so long to work?

Because they work by changing which genes are active and how cells behave over time, not by removing something on the surface. Skin-cell turnover and collagen rebuilding happen over weeks to months, so the visible results — smoother texture, fewer breakouts, softened lines — build slowly. The trials behind the famous claims ran for months, which is the honest timeline to judge a retinoid on.