Not all acids work the same. If you’ve been staring at two bottles in the skincare aisle, one with lactic acid and one with glycolic acid, wondering which one will actually fade those stubborn acne marks, you’re not alone. Both are AHAs (alpha hydroxy acids), both exfoliate, and both get recommended constantly. But they’re genuinely different in ways that matter for your skin, especially when post-acne hyperpigmentation is the thing you’re trying to fix.
I’ve tried both (multiple times, multiple formulas, with varying levels of patience), and the difference in how my skin responded was real. Not dramatic overnight stuff, but the kind of difference that builds over weeks and eventually makes you go “wait, when did that dark spot get lighter?”
The Molecule Size Thing Actually Matters
You’ll see this mentioned everywhere, and for good reason. Glycolic acid has the smallest molecular size of any AHA. That means it penetrates the skin faster and deeper. Lactic acid has a larger molecule, so it sits closer to the surface and works more gradually.
In practical terms: glycolic acid is the stronger, more aggressive option. It gets into the skin quickly, dissolves the bonds between dead skin cells, and speeds up cell turnover at a deeper level. Lactic acid does the same general thing, but more slowly and gently because it doesn’t penetrate as far.
For acne marks specifically, this matters because hyperpigmentation sits at different depths depending on how it formed and your skin tone. Glycolic acid can reach deeper pigmentation, but it can also irritate more easily. Lactic acid takes longer to show results, but it’s less likely to cause the redness, peeling, or sensitivity that can actually make dark spots worse (because inflammation can trigger more pigment production, which is the opposite of what you want).
Which One Is Gentler (Honestly)
Lactic acid. It’s not even close. And I say this as someone who spent a solid year thinking my skin could “handle” glycolic acid at higher concentrations. It could not. My skin told me in the form of angry red patches that took weeks to calm down.
Lactic acid has a natural affinity for the skin’s moisture barrier. It’s actually a component of the skin’s natural moisturizing factor (NMF), which means it hydrates while it exfoliates. Glycolic acid doesn’t have that same moisturizing quality, so it can feel drying, especially at concentrations above 8-10%.
If your skin is sensitive, reactive, or on the darker side (more on that in a second), lactic acid is almost always the safer starting point. If your skin is oily, resilient, and has handled acids before without drama, glycolic acid might give you faster visible results.
For Hyperpigmentation: The Skin Tone Factor
This part doesn’t get talked about enough. If you have a deeper skin tone (Fitzpatrick types IV-VI), your skin produces more melanin in response to irritation. That’s just how melanocytes work. Any treatment that causes inflammation, even “good” inflammation from exfoliation, carries a risk of post-inflammatory hyperpigmentation (PIH).
Glycolic acid’s deeper penetration means a higher irritation risk, which means a higher risk of triggering new pigmentation in darker skin tones. Multiple dermatological studies, including research published in the Journal of the American Academy of Dermatology, have noted that lactic acid tends to produce better outcomes for hyperpigmentation in skin of color because it achieves exfoliation with less irritation.
Lactic acid has also been shown to directly inhibit tyrosinase, the enzyme responsible for melanin production. Glycolic acid can indirectly help with pigmentation by speeding up cell turnover (pushing pigmented cells to the surface faster), but it doesn’t have that same direct effect on the pigment-production pathway.
How to Actually Use Them for Acne Marks
Whichever one you choose, consistency matters more than concentration. A lower percentage used regularly will outperform a high percentage used sporadically (or abandoned because it wrecked your moisture barrier).
For lactic acid, start with 5-10% and use it 2-3 times per week at night. After a few weeks, if your skin is tolerating it well, you can increase to every other night or even nightly. Products like The Ordinary’s Lactic Acid 5% or 10% are affordable and effective.
For glycolic acid, start at 5-7% and use it twice a week maximum. Build up slowly. The Ordinary’s Glycolic Acid 7% Toning Solution is popular, but even that can be too much if you jump straight to daily use. If your skin feels tight, stings, or looks red the morning after, scale back.
With both acids, sunscreen is non-negotiable. AHAs increase photosensitivity, and UV exposure is the single fastest way to darken existing acne marks. All your exfoliating work gets undone if you skip sun protection.
Choosing the Right One for Your Situation
If you’re dealing with mild to moderate acne marks, sensitive or reactive skin, or a deeper complexion, lactic acid is your better bet. It’s slower but steadier, and the hydrating properties mean you’re less likely to compromise your skin barrier in the process.
If you have lighter skin that tolerates active ingredients well, and your marks are stubbornly persistent, glycolic acid’s deeper penetration could give you the push you need. Just respect the adjustment period and don’t rush concentration increases.
You can also alternate between them. Some people use lactic acid on sensitive weeks (around their period, during weather changes) and glycolic acid when their skin feels more resilient. There’s no rule that says you have to commit to one forever.
And if you’re comparing lactic acid to other gentle exfoliating options, mandelic acid is another AHA worth knowing about, particularly if even lactic acid feels like too much for your skin.
Beyond the Bottle
Acids are one piece of the puzzle. Fading acne marks also depends on sleep (that’s when your skin does most of its repair work), managing stress (cortisol triggers inflammation and slows healing), and general nutrition. A solid routine with affordable targeted products combined with patience will get you further than one expensive product used impatiently.
The honest timeline for fading acne marks with AHAs is 6 to 12 weeks for noticeable improvement, sometimes longer for deeper pigmentation. That’s not a failure of the product. That’s just how long skin cell turnover takes. Take photos monthly so you can actually see the progress, because gradual change is hard to notice in the mirror.
Whichever acid you pick, the fact that you’re being intentional about treating your marks instead of just hoping they’ll disappear on their own puts you ahead. Start gently, stay consistent, wear your sunscreen, and give it time. The results will come.