How retinoids convert, and what each step costs you

Skin does not respond to retinol. It responds to retinoic acid. Everything sold as a cosmetic retinoid is a molecule that skin has to convert into retinoic acid first, and the number of steps in that conversion is the single most useful thing to know about a product before comparing its percentage to anything else.

The pathway runs in one direction:

retinyl esters, then retinol, then retinaldehyde, then retinoic acid

Each step is an enzymatic conversion, and each one is inefficient. A retinyl ester is three steps out. Retinol is two. Retinaldehyde is one. Retinoic acid, which in the United States means a prescription, is already there.

Why the percentages are not comparable

This is where most product comparisons fall apart. A 1% retinol and a 0.1% retinaldehyde look a factor of ten apart on the box. They are not a factor of ten apart in effect, because the retinaldehyde skips a lossy conversion step that the retinol has to make.

Any ranking that sorts retinoids by the number on the front is sorting products that are not measured in the same units. Our own retinoid ranking scores the disclosure of a percentage separately from the form, precisely so the two questions cannot be mistaken for one.

What the extra steps cost

Two things degrade as you move back along the pathway. The first is conversion efficiency, which is why the cosmetic strengths are so much higher than the prescription ones. The second is stability: retinol oxidises readily on contact with air and light, and the further out a molecule sits, the more of the shelf life is spent losing it.

That is why packaging is a real variable here rather than an aesthetic one. An opaque, airless pump protects the number on the box. A clear dropper bottle does not, whatever the number says.

The tolerance trade

Moving one step closer to retinoic acid does not automatically mean more irritation. The comparative work on retinaldehyde puts its tolerance nearer to retinol than to prescription retinoic acid at doses that produce comparable appearance changes. That combination, one step closer with tolerance closer to the gentler end, is the entire argument for the retinaldehyde category and the reason it commands the prices it does.

What this means when you shop

Read the form first and the number second. A brand that names its form precisely, publishes a percentage, and packs it opaque has told you three things. A brand that says “retinoid complex” on the front and nothing on the back has told you nothing, and no percentage comparison can rescue that.