Benzoyl peroxide

Benzoyl peroxide releases free-radical oxygen into the follicle, where anaerobic C. acnes cannot survive. Because the mechanism is oxidative rather than receptor-based, bacteria have never developed meaningful resistance to it[1].

The evidence

The most useful trial is also one of the oldest: 2.5 %, 5 % and 10 % BPO reduced inflammatory lesions equally, while irritation rose with concentration[2]. Start at 2.5 %.

Regulatory note

In the EU and UK it is a medicine, not a cosmetic ingredient. It bleaches towels, and it oxidises retinolRetinolStrongRetinol · Vitamin A alcoholThe best-evidenced over-the-counter retinoid for photoageing; now capped at 0.3 % in EU leave-on products.Open in the Index → and ascorbic acidAscorbic acidStrongAscorbic Acid · Vitamin C (L-ascorbic acid)The active form of vitamin C; well-evidenced for photoageing at 10–20 % and low pH, but unstable.Open in the Index → on contact — use it at a different time of day. adapaleneAdapaleneStrongAdapalene · Synthetic third-generation retinoidA prescription-strength retinoid available OTC at 0.1 % in the US; stable, well tolerated, strong acne evidence.Open in the Index → is the exception; the two are chemically compatible.