Glycolic Acid

Glycolic acid is an alpha hydroxy acid, or AHA, originally derived from sugar cane though now almost always made synthetically. It is the smallest molecule in the AHA family, which is the fact that governs everything else about it: a smaller molecule passes through the surface of the skin more readily, so glycolic acid tends to work faster and more noticeably than its larger relatives, and to be less forgiving with it.

On an ingredients list it appears as Glycolic Acid. In over-the-counter products it usually sits between 5% and 10%, with some leave-on treatments going higher and professional peels considerably higher still. Two products at the same percentage can behave very differently depending on pH, since an AHA needs a reasonably low pH to be active, and on whether the acid is partly neutralised in the formula.

You will find it in toners, pads, leave-on serums, overnight treatments, cleansers and body lotions. Cleansers deliver comparatively little simply because the contact time is so short. It also appears widely in body products aimed at rough, dry-looking skin on the arms, legs and feet, where skin is thicker and tolerates it more easily than the face.

Of all the exfoliating acids it has the longest research history behind it, which is part of why it remains the default AHA in both retail products and clinic treatments.

It is also a humectant in its own right, which is a detail people often miss: alongside the exfoliation, it draws a little water into the skin, so a well-formulated glycolic product need not leave skin feeling stripped.

The outermost layer of skin is made of dead cells held together by bonds that normally break down on their own so the cells can shed. That process becomes slower and less even with age, with sun exposure and in dry conditions, and the result is skin that looks dull, rough and uneven, with makeup sitting poorly on top. Glycolic acid works by weakening those bonds so the surface sheds more evenly and more readily.

Because it is small enough to move through the surface efficiently, it does not stop at simple polishing. Consistent use appears to prompt activity in the deeper layers too, which is the basis of its long-standing association with the look of firmness and the appearance of fine lines, rather than just immediate smoothness. It also helps disperse the surface pigment that makes marks look defined, so tone gradually looks more even.

The trade-off is directly linked to the same property. Removing surface cells more quickly leaves skin temporarily more exposed, and glycolic acid is well known to increase sensitivity to daylight. It also stings more readily than gentler acids on skin that is dry or already compromised. Used sensibly, with sunscreen and without stacking other actives on top, that is manageable; used enthusiastically, it is the fastest route to an irritated barrier of any common cosmetic ingredient.

Glycolic acid is effective and unsubtle. Restraint is what makes it work well.

• Start low: 5–7% is plenty to begin with. Move to 8–10% only once skin is clearly comfortable.
• Frequency: once or twice a week at first, building slowly. Very few people need it nightly, and nightly use is where most problems begin.
• When: evening, on clean, dry skin. Wait a minute or two before the next step.
• Order in the routine: after cleansing, before serums and moisturiser. If you use a leave-on toner form, it goes first.
• Pairs well with: niacinamide, hyaluronic acid, panthenol, ceramides and a good moisturiser. Barrier support makes a real difference to how comfortable it feels.
• Keep apart from: retinoids, vitamin C, benzoyl peroxide, other acids and any scrub. Alternate nights, and never layer acids to speed things up.
• SPF 30 or above every morning without exception. Glycolic acid increases sensitivity to daylight, and skipping sunscreen undoes the tone benefits entirely.
• Avoid the eye area and go carefully around the nostrils and mouth.
• Consider lactic acid instead if your skin is dry, thin or easily reddened. It does a similar job more gently.
• Stop if skin feels tight, shiny or stings with water. That is over-exfoliation. Rebuild with plain moisturiser before restarting.

Smoothness and radiance often appear within a couple of weeks; tone and fine lines take two to three months.