There are topical wash/cleansers that are useful for the Acne Vulgaris of your back. Treatment for the acne will depend on its severity.
There are creams available to be applied over the acne in the day as well as night for acne prone skin that will help in its reduction in size and numbers.
However, if the acne is moderate in nature, Oral antibiotics will be useful, together with the creams. Isotretinoin (the Vitamin A medicine) is reserved for severe acne and has to be used with caution, if necessary.
Hope this helps.
Warmest Regards