Thank you for your question. Firstly, I think it is important to confirm the diagnosis of acne as there are other conditions that can appear on the arms and back which can mimic acne such as keratosis pilaris and pityrosporom folliculitis. The management would be dependent on the diagnosis.
If it is indeeed acne, topical treatments like tretinoin cream and topical clindamycin may be helpful. Oral antibiotics and isotretinoin can be considered in more stubborn cases.
I would suggest you see your doctor/dermatologist to confirm the diagnosis so that the most appropriate treatment can be given.
