How can 5% and 10% concentrations of benzoyl peroxide, be less effective than the 2.5%?

Question

Herbert P. Goodheart, MD 1971 (Hahnemann Medical College). Acne For Dummies (2006). p. 234.

Benzoyl peroxide is the most commonly used over-the-counter acne medication, and is also the most effective medication for treating PFB. It comes in varying strengths ranging from 2.5 to 10 percent. There’s no proof that anything higher than 2.5 percent works any better than the higher concentration, and the lower concentrations are cheaper and may be less irritating to your skin. Clear By Design, Clearasil, Fostex, Neutrogena, and Noxzema are just a few of the benzoyl peroxide brand names available.

The statement beneath from this abstract feels paradoxical.

The 2.5% benzoyl peroxide formulation was more effective than its vehicle and equivalent to the 5% and 10% concentrations in reducing the number of inflammatory lesions (papules and pustules).

Answer 1

You are reading the outcome incorrectly, the 5% and 10% formulations are not less effective, they are equally effective.

The specific text is

The 2.5% benzoyl peroxide formulation was more effective than its vehicle and equivalent to the 5% and 10% concentrations

I don't know how you read that as the higher concentrations are less effective.

The 2.5% formulation was more effective that its vehicle, meaning the solution it was added to.

The correct reading is that whatever mechanism benzoyl peroxide works by is essentially saturated at 2.5%, and a higher concentration is simply wasted.