A larger, slower-penetrating AHA often chosen for sensitive or darker skin. Used in acne peels and now in deodorants that lower skin pH.
Mandelic acid is an AHA with a bulky ring structure, so it penetrates more slowly than glycolic acid and tends to sting less. It also has some antibacterial activity. Clinics use it in peels, often combined with salicylic acid, for acne and pigmentation, especially in darker skin where irritation-driven darkening is a concern.
In body care it appears mainly in 'whole-body' deodorants and washes, where the acid is meant to lower skin pH and slow the bacteria that cause odour rather than to exfoliate.
Evidence comes mostly from small peel studies. In a 44-patient comparison, a 20% salicylic–10% mandelic acid peel beat a 35% glycolic peel for active acne and post-acne dark marks with fewer side effects; a Cochrane review, looking at participant-rated improvement, found little difference between the two peels and rated the evidence low quality. Leave-on mandelic products and mandelic deodorants have little published testing.
44 patients in India with acne and post-acne marks had six fortnightly peels of 35% glycolic acid or 20% salicylic–10% mandelic acid; both helped, and the salicylic-mandelic peel worked better on active acne and pigmentation with fewer side effects.
49 trials (3,880 people): azelaic acid is probably less effective than benzoyl peroxide and about as effective as tretinoin for acne; evidence for salicylic acid, nicotinamide and alpha-hydroxy acids was low or very low quality, from small head-to-head trials, and no conclusions were reported for zinc or sulphur.