Can I use clearasil (benzoyl peroxide) on my back acne?

Yes. However, you will probably get a much better result with a prescription topical agent such as Clindamycin with or without a retinoic acid derivative. If this isn't sufficient, your physician may choose an oral antibiotic that's right for you -- oral tetracycline kept me mostly clear for 25 years. Dermatologists have bigger guns. Today, no one should have to endure problem acne. Good luck.