Benzoyl peroxide is one of the few acne ingredients bacteria cannot become resistant to — it works by releasing oxygen, which acne bacteria simply can't survive. That's why dermatologists pair it with antibiotics like clindamycin or doxycycline: it keeps the antibiotic working over time.
The 4% version is the smart pick. Studies show 4% is about as effective as 10%, with considerably less dryness and irritation. Higher isn't better here — it just burns more.
As a wash rather than a leave-on, contact time is short, which further reduces irritation while still delivering the benefit. Good for the face, and genuinely excellent for chest and back acne.
Cheap, available everywhere, and it works.