Retinaldehyde

Retinaldehyde has no activity of its own. It does nothing until skin converts it to retinoic acid, which then binds retinoid receptors and turns up collagen production, speeds turnover of the outer layer, and slows pigment production.

Two details from the research are worth knowing because they cut against the marketing.

Most of what you apply gets stored, not converted. Work from the group that developed topical retinaldehyde found that skin takes it up and then mostly converts it into retinyl esters — a storage form — releasing relatively small amounts of retinoic acid from a large reservoir. That slow trickle is a plausible explanation for why retinal is so much better tolerated than tretinoin: less active retinoic acid arrives at any one moment.

The gap to tretinoin is large. In animal skin, an equal dose of retinoic acid produced 25 times more active retinoic acid in the tissue than retinaldehyde did. The biological effects were qualitatively similar. The quantities were not.

It also has an antibacterial property retinol does not. In laboratory testing of all three forms side by side, only retinaldehyde showed meaningful antibacterial activity, and applying 0.05% retinaldehyde to volunteers' foreheads for two weeks measurably reduced acne bacteria on the skin. Interestingly, the researchers traced this to the aldehyde chemical group rather than to anything retinoid — unrelated aldehyde compounds did the same thing. It is a real finding, but it is a measurement of skin bacteria in healthy volunteers, and no study has shown it drives improvement in actual acne.

The retinoid ladder, from weakest to strongest, with how many conversion steps each needs:

• Retinyl esters (retinyl palmitate, retinyl acetate) — storage forms. Three steps away. The weakest.
• Retinol — two steps away. The most widely studied cosmetic retinoid.
• Retinaldehyde — one step away.
• Retinoic acid (tretinoin, Retin-A) — the active molecule itself. Prescription in most countries.

Two other cosmetic retinoids get compared with retinaldehyde:

• Adapalene — a synthetic prescription-strength retinoid, available without prescription at 0.1% in the US for acne. Not on this conversion ladder; it binds receptors directly.
• Hydroxypinacolone retinoate (HPR, "Granactive Retinoid") — a retinoic acid ester claimed to work without conversion. Whichever way the mechanism argument goes, retinaldehyde has far more published human evidence behind it.

• 0.05% — the workhorse concentration in the published research. Nearly every photoaging, rosacea and antibacterial study used it.
• 0.1% — the other studied strength, used in the acne research and in most modern serums. Two studies compared 0.05% and 0.1% head to head and found little difference on most measures, with 0.1% doing better on wrinkles in the longer of the two.
• 0.5% — dose-ranging only, slightly less well tolerated.
• 1% — worth knowing about: in a 229-person study, only about 70% of people tolerated it. This is not a strength to chase.
• 0.2% and above now appears on the market. That is above anything tested in the controlled trials.

Both 0.05% and 0.1% were described as well tolerated on facial skin over periods of up to three years.

Give it three months. In the longest controlled trial, wrinkle and roughness measurements improved significantly by week 18. The same trial found the effect was less pronounced by week 44 than at week 18, which is a result worth knowing and one that rarely gets repeated.

Tolerability compared with tretinoin is retinaldehyde's strongest, best-evidenced claim. In a 355-person comparison over 44 weeks, tretinoin produced redness in 44% of people, scaling in 35% and burning or itching in 29% during the first four weeks. All three were significantly less common with retinaldehyde.

Compared with retinol, the picture is genuinely unresolved. The one direct tolerability comparison found retinol and retinaldehyde equally low in irritation overall — but retinaldehyde produced more scaling than retinol. That study included only six people, which is far too few to settle anything either way.

Retinaldehyde is chemically fragile. In one laboratory test, free retinaldehyde in water lost more than 96% of its content in seven days — stored cold, and in the dark. That is why these products come in opaque tubes and airless pumps, and why formulators go to considerable trouble with encapsulation and oil-based formulas.

A common piece of advice is to judge freshness by colour. Retinaldehyde is naturally yellow-orange, but no study supports using colour as a stability guide — and the degradation experiment above happened with no light at all. Colour is not a reliable indicator either way.

There is one genuinely reassuring safety finding. Volunteers applied 7 mg of retinaldehyde daily to 40% of their body surface for two weeks — a deliberately extreme exposure — and no change appeared in any vitamin A measure in their blood, with no retinaldehyde detectable at all.