Mandelic Acid
In plain terms
An alpha hydroxy acid from bitter almonds, with the largest molecule of the common AHAs, so it works more slowly and gently. That makes it a favorite for sensitive and darker skin, but like every AHA it still makes skin more sun-sensitive.
What the evidence shows
Mandelic acid is an alpha hydroxy acid, the same exfoliating family as glycolic and lactic acid, but its molecule is the biggest of the bunch. That size means it soaks in slowly, which is why it stings and reddens less and gets recommended for sensitive, acne-prone, or deeper skin tones where harsher acids can leave marks. The gentleness is real, but it does not buy an exemption from the one effect that defines AHAs: by loosening the dead surface layer, it leaves skin more reactive to the sun. Europe's cosmetics committee confirmed this is a class trait of AHAs, raising sunburn-cell formation and lowering the dose of UV it takes to burn, so daily sunscreen is part of the deal here just as it is with glycolic acid. One honest gap: unlike glycolic and lactic acid, mandelic acid has never had its own dedicated safety assessment from the CIR or the EU panel, so it inherits the AHA-class conclusion rather than carrying an individual one. The practical safety story is the familiar AHA one: dose and pH decide how strong it is, overdoing any exfoliant leads to irritation, and the sun sensitivity is reversible once you stop.
On the label
Listed as mandelic acid or sometimes just AHA. Treat it like any acid exfoliant: ease in, do not stack it with other strong acids or retinoids at first, apply at night, and use morning sunscreen. Its slower penetration makes it a reasonable starting point if glycolic acid has been too harsh.