Retinoid, retinol, retinal: the only difference that matters is conversion
Three words used interchangeably on packaging, describing molecules that are between one and three enzymatic steps away from doing anything at all.
Every retinoid that works in skin works as the same molecule: retinoic acid, binding a nuclear receptor and changing which genes a keratinocyte transcribes. Everything sold on a shelf is a precursor that has to become retinoic acid first, and the entire practical difference between products is how many enzymatic steps that takes.
The ladder
- Retinyl esters (retinyl palmitate, retinyl acetate) — three steps away. Extremely gentle, which is a polite way of saying most of it never arrives.
- Retinol — two steps. The workhorse of the shelf.
- Retinaldehyde (retinal) — one step. Faster, more irritating than retinol at equivalent labelled strength, considerably less irritating than the prescription.
- Tretinoin, adapalene, trifarotene — zero steps. Prescription. Already the active form, or a synthetic that binds the receptor directly.
Each step is enzyme-limited, and the enzymes saturate. This is why doubling a retinol percentage does not double the effect, and why 1% retinol is not “equivalent to” a prescription in anything but marketing copy.
Why adapalene is the one we reach for first in acne
Adapalene is a synthetic that binds the retinoic acid receptor selectively, is stable in light and in benzoyl peroxide, and has a substantially better irritation profile than tretinoin at comparable efficacy for comedonal acne. In the UK it is available without prescription at 0.1%. For a great many people arriving here having spent £180 on a serum, it is a £12 tube that will outperform it.
We still see people started on 0.05% tretinoin who abandon it in three weeks because their face peeled. That is not a tretinoin failure. That is a dosing failure.
The purge, and what it actually is
Retinoids accelerate follicular turnover, so microcomedones that were going to become inflammatory lesions over eight weeks become them over two. It looks like the product is causing acne. It is, in a narrow sense, revealing it early.
It should settle within six to eight weeks. If it is still going at twelve, something is wrong — either the concentration, the frequency, or the diagnosis.
How to start, if nobody has told you
Twice a week, pea-sized, on a dry face, at night, with a plain moisturiser over the top. Build to alternate nights over six weeks, and to nightly only if your skin says yes. Wear sunscreen, not because retinoids make you burn dramatically faster, but because you are spending money to remodel skin that ultraviolet is simultaneously taking apart.