AHA is one of those acronyms that appears on half the products in a bathroom without ever being properly explained. Here's what alpha hydroxy acids are, how the main ones differ, and how to choose between them.
What AHAs do
Your skin sheds dead cells continuously, but not always efficiently — and when they linger, skin looks duller and feels rougher than it should. Alpha hydroxy acids loosen the bonds holding those surface cells together, so they lift away more readily. The result is skin that looks brighter and feels smoother, with products applied afterwards absorbing more easily.
AHAs are water-soluble, which means they work across the surface. That's the fundamental difference from salicylic acid, a BHA, which is oil-soluble and gets into the pore lining instead.
The main AHAs
They're distinguished largely by molecule size — smaller molecules penetrate faster, which means more potency and more potential for irritation.
- Glycolic acid — derived from sugar cane, and the smallest AHA molecule. The most potent and the most researched, best suited to skin that's already accustomed to acids.
- Lactic acid — a larger molecule, so gentler, with the useful quirk of being a humectant too: it hydrates while it exfoliates. The sensible starting point for most people.
- Mandelic acid — larger still, and the gentlest of the three. Worth considering for reactive skin.
Choosing a strength
Cosmetic formulas typically run from around 5% to 10%, occasionally higher in weekly treatments. As with all acids, the format changes the equation: a rinse-off cleanser at 10% is gentler than a leave-on serum at 5%.
Start low, start infrequent — twice a week is plenty at first — and build only if your skin is comfortable. A brief tingle is normal; stinging, persistent redness or flaking is not.
How to use them
Evening, on clean dry skin, followed by moisturiser. Don't combine them with retinol in the same routine while you're building tolerance — alternate nights instead.
The important part: AHAs increase skin's sensitivity to sunlight, and that effect persists for about a week after use. Daily SPF isn't optional if you're using them — without it you're working against yourself.
Pair them with hydration. Hyaluronic acid, ceramides and squalane all help keep skin comfortable while you exfoliate.
How often is too often?
More exfoliation is not better exfoliation. Skin that's been over-exfoliated looks shiny in a tight, waxy way rather than genuinely radiant, stings when products are applied, and often becomes more reactive over time. If that sounds familiar, stop all acids for a fortnight and rebuild with a plain moisturiser — skin recovers well when left alone.
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