I had a food allergen with pork. Once I stopped eating pork my acne cleared up. I'm working on what other food(s) I might be allergic to as well.
Changing my diet cured my back acne after about 12 years of suffering. It was pretty bad, especially in winter, occasionally I had cysts on my back, and even two surgeries to remove them with nasty scars left, but I am a guy, and do not care much of the scars. I visited doctors, tried medications, and of course it didn't work well, because acne at an adult age is a sign of internal disorder, and it can not be cured by tablets or gels. Finally I went through a cleaning program by myself (colon and liver), stopped esting any artificial food, and also easy digested carbohydrates (bread, cookies, sugar, portatoes and so on), and my acne virtually disappered in some months. I have again it only if violate my diet.