This is because of sensitivity to Benzoyl peroxide .it is better to start Benzoyl
peroxide at low concentration to avoid side effects like swelling ,itching with the passage of time tolerance will develop and then high dose of Benzoyl peroxide to achieve high tolerance level to reduce the no bacterial count.
One thing which is very good about Benzoyl peroxide is that it bacteria Propionibacterium acnes do not develop resistant against it. read more at pharmacy online .