Why does Roaccutane work for some and not for others? Furthermore, how much of a role does diet play with acne?

I have mild acne on my face (generally around my mouth and very occasionally on my cheeks and forehead). I have been prescribed Roaccutane twice. Friends of mine have been "cured" of acne following one course of Roaccutane. I'm not willing to take it again nor am I willing to take antibiotics and develop a resistance. I need help in determining the cause of acne and whether I can make certain lifestyle changes to cure it. Acne is a confidence killer in the corporate environment. Please help

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