Tetracycline for Acne

I want to ask a question about the dosage of tetracycline for acne. I am using tetracycline 500 mg B.I.D for 3 weeks now and my acne is already healing. I want to know if how long i can take tetracycline, because the first time i take it it heals my acne then after i heal i stopped then after 2 months of stopping it comes back again, now im using it again but im sfraid to stop because my acne might comes back again. I want to know whats the right thing to do. thank you very much.