Although my acne medication works on my face, I've had the oddest problems with back and chest acne! It's driving me nuts! I've tried the following changes to help get rid of it, if you have any suggestions please assist!!
*drinking more water
*changing my diet (eating less sweets and salty stuff)
Are you washing your back with a scrub brush each day?
Try doing that along with using a medicated cleanser called Salac. It is made by Medicis and I found it over 10 years ago and it works wonderfully! There is no prescription required and you should be able to find it at any pharmacy. They usually either keep it behind the counter or they'll order it for you. If you're lucky enough to live near a Drug Emporium, they keep it stocked on the shelf.
Be sure and get the wash, not the foam, and use it with your scrub brush when taking a shower.
I don't know if it is especially for back acne, but that is why my derm had me start on it many years ago. On the bottle it says "Acne Medication Cleanser". I had it on my back, shoulders and chest. Its really the only thing that consistently works for me.