Thanks for writing in.
The condition is probably unrelated to opiates as drug allergy
will not last for such long period.It presents acutely after giving drugs and subsides in few days.
A fungal infection
or contact dermatitis
over back can cause itching and erosions over back.A visit to your primary physician will help to diagnose and treat the condition.
Till the time apply a good quality moisturizer over the area twice daily and take tab cetirizine
5mg once or twice a day.
Hope this helps.