Glycolic Acid (AHA)

What it does

The smallest AHA molecule, so it penetrates fast: smooths texture, brightens dullness and helps fade surface marks. Effective - and easy to overdo. Gentler cousins: lactic acid (hydrates while it exfoliates) and mandelic acid (mild, good for sensitive or acne-prone skin).

How to use it

Evenings, two or three nights a week to start. SPF the next morning is non-negotiable - exfoliated skin is more sun-sensitive.

What not to mix it with

• retinoids (same night) - see Retinoid + AHA
• vitamin C (same session) - see Vitamin C + AHA
• BHAs (same session) - see AHA + BHA
• copper peptides - see Copper peptides + AHA
• kojic acid (separate strong products) - see Kojic acid + AHA

What it gets along with

Fine with hydrators and barrier moisturizers - they're how you keep an AHA habit comfortable.