Lactic acid: the difference showed up not while using it, but **after stopping**

A 1996 22-week, double-blind, vehicle-controlled randomised trial at Massachusetts General Hospital. Seventy-four women aged 40 to 70 with moderately severe photodamage applied 8% glycolic acid, 8% L-lactic acid or vehicle cream twice daily to face and forearms.

On the face, the proportion improving by at least one grade in overall photodamage severity was 71% for lactic acid, 76% for glycolic acid and 40% for vehicle (P < .05).

The forearms are more interesting. At week 22, lactic acid beat vehicle on four measures: overall severity (P < .05), mottled hyperpigmentation (P < .05), sallowness (P < .05) and roughness. In the same trial, glycolic acid beat vehicle on two — overall severity and sallowness.

Same trial, same concentration, same duration, and on the forearms lactic acid won on more measures. A count of outcomes does not tell you effect size, but it does show that 'the AHAs are all much the same' is not a safe assumption.

The authors' wording for the conclusion was the same for both: modestly useful. The 40% of the vehicle group who also improved belongs in that reading.

This trial also did not report pH.

The 1983 trial has the most to teach on this page. A 21-day, randomised, double-blind, contralateral study in sixty people with moderate to severe xerosis compared three things: a specially neutralised 12% lactate lotion, a 5% lactic acid lotion, and a non-lactated emollient lotion.

Measurements were taken on days 0, 7, 14 and 21 — and then again on days 28 and 35, after treatment stopped.

The first result is this: all three significantly reduced xerosis severity scores. The plain emollient included.

The difference appeared in the regression period after stopping.

• Against the plain emollient, 12% lactate held severity scores significantly lower on the lateral calf at days 28 and 35, and on total severity at day 35.
• Against 5% lactic acid, 12% lactate was significantly better on total severity at days 28 and 35.

How to read this: if you only look at improvement during use, the three products are indistinguishable. Apply any moisturiser consistently and dryness scores fall. What the ingredient contributed showed up in how long the improvement held after stopping.

Given that cosmetic trials usually end on the last day of treatment, this design is rare and valuable.

The 2002 trial compared 10% lactic acid with 12% ammonium lactate in foot xerosis over four weeks. Both improved significantly at two and four weeks, with no statistically significant difference between them. Note, though, that 18 of the 53 enrolled were excluded from analysis. A result missing a third of its participants deserves to be read with corresponding caution.

The 1997 trial rotated four acids at 8% (glycolic, lactic, tartaric, gluconolactone), vehicle and an untreated site randomly across the arms and forearms of eleven healthy people for four weeks. At week 4, the sites were challenged with a surfactant and water loss and erythema were measured.

The sites with significantly less barrier damage after the challenge were gluconolactone and tartaric acid. Both beat vehicle and untreated skin significantly (P < .01).

Lactic acid was not one of them. It sits with glycolic acid on the other side.

The authors read the effect as more marked in the molecules with antioxidant properties.

Eleven people cannot settle it. But it is worth writing down that the humectant character lactic acid showed in the item above is a different thing from defending the barrier. Holding water and shielding against an irritant are not the same job.

Lactic acid has a single hydroxyl group on the alpha carbon. It is an alpha hydroxy acid in the literal sense, and there is no dispute about the classification.

So the wording in Annex 1, item 2(b)(9) of the Regulation on Precautions for Use and Labelling of Allergenic Fragrance Ingredients applies to it directly.

Above 0.5%:

• Use with sunscreen, as it may increase the skin's sensitivity to sunlight (rinse-off and hair products excepted)
• Test on a small area first to check for skin reaction

Above 10%, or at a pH below 3.5, one more line:

• Consult a specialist, as the high AHA content carries a risk of adverse effects

The usefulness is the same as described on the glycolic acid page: even when a company does not disclose the percentage, the presence of that third line means above 10% or below pH 3.5.

By contrast, gluconolactone does not carry this text. That page explains the situation.