Walk into any UK pharmacy and you'll see retinol, retinal and retinoid sitting on adjacent shelves at wildly different price points. Marketing teams want you to think the spelling difference is cosmetic. Dermatologists disagree, and the conversion biochemistry backs them up.
How retinoids actually work in the skin
Every topical vitamin A derivative has to convert into retinoic acid before it does anything useful. Retinoic acid is what binds to receptors in skin cells and signals them to behave like younger cells: faster turnover, more collagen, less pigmentation. The catch is that retinoic acid itself is too irritating for most people to tolerate as a daily over-the-counter product, so brands use precursors that the skin converts in steps.
The conversion chain
Retinol needs two conversion steps: retinol → retinaldehyde (retinal) → retinoic acid. Retinal skips the first step, so it's one conversion away from active. Retinyl palmitate, the gentlest ester, needs three steps and loses most of its potency along the way.
Why retinal often outperforms retinol at the same percentage
A 2021 review in the Journal of Cosmetic Dermatology pooled the available clinical data and found 0.05% retinal produced visible improvements in fine lines and pigmentation in roughly half the time of 0.5% retinol. The conversion efficiency explains it: less is lost along the way, so a lower percentage on the label translates to a comparable dose at the receptor.
That doesn't mean retinal is always the right pick. Tolerance varies enormously. Some users who couldn't get past 0.3% retinol without flaking handle 0.1% retinal fine; others react to retinal precisely because more of it reaches the active form. If you flushed and peeled on tretinoin, you'll likely react to retinal too.
Picking between them in practice
Beginners with no retinoid history
Start with retinol at 0.1-0.3%, twice weekly for a fortnight, then alternate nights. La Roche-Posay Retinol B3 Serum is the workhorse most UK dermatologists default to: cheap, well-formulated, and the niacinamide buffers irritation.
Plateaued on retinol
If you've been on 0.5-1% retinol for six months and the results have stalled, retinal is the logical next step before prescription tretinoin. Medik8 Crystal Retinal 3 (UK formulation) is the entry point most people tolerate.
Sensitive skin or rosacea
Stay on retinaldehyde derivatives or hydroxypinacolone retinoate (HPR). Both reach the receptor with less of the inflammatory cascade that classical retinol triggers.
What the percentage actually means
A 1% retinol product does not contain 1% retinoic acid. After conversion losses and the skin's own metabolism, you're looking at something closer to 0.05-0.1% effective dose at the receptor. This is why direct comparisons between 'percentages' across product categories are meaningless. The only honest comparison is clinical outcome at week 12, and most brands don't publish that data.
Pick by tolerance and goal, not by the bigger number on the label. Sunscreen every morning is non-negotiable on any retinoid: skin is more photosensitive for at least the first month, and pigmentation rebounds without SPF protection.