Mandelic acid is an alpha hydroxy acid originally derived from bitter almonds. It is the largest of the commonly used AHAs, and almost everything distinctive about it follows from that.
Size and speed
Where glycolic acid has two carbon atoms, mandelic acid carries a benzene ring and is several times larger. Larger molecules move through the stratum corneum more slowly and less deeply at equivalent concentration.
That makes mandelic acid the gentlest of the group and the slowest to show results. It is a genuine trade-off rather than a formulation trick, and it is why mandelic acid is often suggested for sensitive skin, for skin new to acids, and for deeper skin tones where aggressive exfoliation carries a risk of post-inflammatory pigmentation.
Partly oil-soluble
Unusually for an alpha hydroxy acid, mandelic acid is somewhat lipophilic. Glycolic and lactic acid are strictly water-soluble and work on the surface. Mandelic acid's benzene ring gives it some affinity for oil, so it has a limited ability to work within a sebum-filled follicle.
This puts it partway between a typical AHA and salicylic acid, which is why it appears in formulations aimed at oily and congested skin rather than purely at surface texture.
Antibacterial activity
Mandelic acid has documented antibacterial properties, and a related compound, methenamine mandelate, has a history of medical use in urinary tract infections. Whether that activity is meaningful at cosmetic concentrations on skin is less established than the exfoliating effect.
Almond allergy
A reasonable question, since the acid derives from bitter almonds. Mandelic acid is a single isolated molecule rather than an almond protein extract, and almond allergy is a protein-mediated response. Most cosmetic mandelic acid is also now synthesised. Anyone with a severe nut allergy should still take medical advice before use.
Sun sensitivity
Like all alpha hydroxy acids, mandelic acid thins the outermost layer as it works, increasing UV sensitivity. Daily broad-spectrum SPF is required alongside it.
