Glycolic acid, explained

Glycolic acid is an alpha hydroxy acid derived from sugar cane, and it has the smallest molecule of any AHA in common use. That is not trivia — molecular size determines how readily it penetrates, and glycolic acid's smallness is precisely why it is both the most effective AHA on surface texture and the most irritating. It works by loosening the bonds between dead cells on the skin's surface so they shed more readily, which over weeks produces the smoother, brighter, more even surface people buy it for.

Glycolic, lactic, mandelic: the size ladder

The three AHAs you will meet are best understood as a size gradient. Glycolic is smallest — fastest, strongest, most irritating, best for dull texture and sun damage on robust skin. Lactic is larger, gentler, and more hydrating, because it is also a humectant; it is the sensible choice for dry or slightly reactive skin. Mandelic is largest and gentlest, penetrating slowly enough that it suits skin that reacts to everything, including deeper skin tones where post-inflammatory pigmentation is a concern. Trading down this ladder costs you speed and buys you comfort, and for most people that is a good trade.

The strengths that matter

• 5% — an entry-level leave-on, or a daily toner. Gentle enough for beginners and for skin that has never seen an acid.
• 7-10% — the mainstream leave-on range, and where most visible results on texture and tone happen. This is the strength of the popular exfoliating toners.
• 10-30% in a rinse-off peel — weekly at most. The rinse-off format is what keeps a high percentage tolerable.
• Above 30% — professional treatment only. Not something to buy online for home use, regardless of who on the internet is doing it.

One nuance the marketing rarely mentions: the pH of the formula matters as much as the percentage. A 10% glycolic acid at pH 4.5 is meaningfully milder than a 10% at pH 3.2, because more of the acid is in its free, active form at lower pH. Brands that publish pH are telling you something useful about themselves.

What it does well

Dullness, rough surface texture, uneven tone and the shallow discoloration left behind by old breakouts — these are glycolic acid's territory, and it is genuinely good at them. It also improves how well everything applied after it absorbs, and there is reasonable evidence that regular AHA use supports collagen over the long term. If your complaint is "my skin looks tired and rough," this is the ingredient. If your complaint is "my pores are full of blackheads," you want salicylic acid instead — it is oil-soluble and glycolic acid is not.

The sunburn warning, in the FDA's own words

This is the part most guides skip, and it is the most important paragraph on this page. The FDA has considered evidence that topically applied cosmetics containing AHAs may increase the skin's sensitivity to the sun while the products are used and for up to a week after use stops, and that this may increase the possibility of sunburn. Government-sponsored studies have determined that applying glycolic acid to the skin makes people more susceptible to sun damage, including sunburn. The FDA's recommended labeling statement reads:

"Sunburn Alert: This product contains an alpha hydroxy acid (AHA) that may increase your skin's sensitivity to the sun and particularly the possibility of sunburn. Use a sunscreen, wear protective clothing, and limit sun exposure while using this product and for a week afterwards."

Note the week afterwards — that is the detail people miss. Stopping your acid before a beach holiday does not clear the sensitivity by the time you land. Daily sunscreen is not optional alongside this ingredient; it is part of the instructions.

How to use it

1. Twice a week to start, at night, on completely dry skin. Damp skin increases penetration and is a common cause of a bad first experience.
2. One exfoliant only. Glycolic acid and salicylic acid and a scrub in the same week is over-exfoliation with extra steps.
3. Alternate with your retinoid rather than layering. Both accelerate turnover; both irritate.
4. Follow with a barrier moisturizer. Exfoliation without replenishing lipids is where the tight, shiny, over-exfoliated look comes from.
5. Sunscreen every single morning — see the warning above.

Who should skip it

Skin that is currently compromised, peeling or reacting. Rosacea-prone skin, which usually does better with azelaic acid. Anyone using a prescription retinoid without having discussed adding an acid with their prescriber. And anyone who cannot commit to daily sunscreen, in which case a glycolic acid routine is actively working against you — you are increasing photosensitivity and doing nothing about it.