“Mandelic acid has a larger molecule size that penetrates skin more slowly, making it gentle yet effective at exfoliating, clearing congestion, brightening, fading post-acne marks and hyperpigmentation, and has antibacterial and antifungal properties that help prevent acne." (Post graphic adds: "Gently exfoliates, clears congestion, and…”

On acne. Dayal 2020 compared a 45% mandelic acid peel against a 30% salicylic acid peel in mild-to-moderate acne vulgaris. Both agents showed roughly equal overall efficacy; salicylic acid performed better on non-inflammatory lesions and mandelic acid performed better on inflammatory lesions, with no significant overall difference in the acne score change. Adverse effects were fewer with the mandelic acid peel. A secondary report of the same trial gives the comedone reduction at 12 weeks as 73.4% for salicylic acid against 59.66% for mandelic acid (P=.044). A separate randomised South Indian study of 40% mandelic versus 30% salicylic reached the same bottom line: equal efficacy across grades I and II acne, with adverse effects higher in the salicylic arm. An earlier RCT (Jartarkar 2017, n=50) found salicylic acid peel more efficacious than mandelic acid peel for non-inflammatory acne, statistically significant. On pigmentation. Sarkar 2016 randomised 90 melasma patients to 35% glycolic acid, a combined 20% salicylic / 10% mandelic acid peel, or a phytic combination peel, every 14 days to 12 weeks. In the salicylic-mandelic group mean MASI fell from 11.85 at baseline to 4.37 at 12 weeks, about a 61% reduction, with 41.67% change retained at 20 weeks. Reported side effects in the glycolic group included burning, erythema, desquamation and transient post-inflammatory hyperpigmentation. Garg 2009 compared glycolic peels against salicylic-mandelic peels in active acne and post-acne pigmentation and concluded both were effective and safe in Indian patients, with the salicylic-mandelic peel better for active acne and post-acne hyperpigmentation. On antibacterial activity. Mandelic acid showed in vitro activity against Staphylococcus aureus, including MRSA, at 40, 80 and 160 mg/mL by disk diffusion, with MIC/MBC of 20/20 mg/mL for the methicillin-sensitive type strain and 40/40 mg/mL for MRSA. This is a petri-dish study on Staphylococcus aureus. No study was retrieved testing mandelic acid against Cutibacterium acnes, the organism implicated in acne, and none testing antibacterial effect on human skin. On antifungal activity. The antifungal literature retrieved for mandelic acid is about synthetic mandelic acid derivatives (1,3,4-thiadiazole thioether and oxadiazothioether compounds, 4-substituted derivatives) screened as agricultural fungicides against plant pathogens including Thanatephorus cucumeris, Gibberella saubinetii, Verticillium dahliae and Sclerotinia sclerotiorum. A further study assessed oxi-acetyl and oxipropionyl mandelic acid derivatives as ointment preservatives. No study was retrieved testing mandelic acid itself against Malassezia or any skin fungus, in vitro or on skin. On the molecule-size mechanism. The QSAR analysis of 106 chemicals measured on human skin identifies several molecular descriptors governing stratum corneum permeability, including lipophilicity and molecular geometry, not molecular weight alone. No study was retrieved that directly measured mandelic acid penetration rate against glycolic acid penetration rate in human skin. The "larger molecule, slower penetration, therefore gentler" formulation was found almost exclusively on brand and retailer pages.