This is a single-center (University Malaya Medical Centre), randomized, double-Blind, placebo-controlled versus bupropion hydrochloride sustained-releases study trial on 70 men who have methadone emergent sexual dysfunction. Bupropion hydrochloride sustained-release (Wellbutrin SR) 150mg once a day for 7 days then 150mg twice a day for 42 days. Taken orally by the subjects.The total duration of the study is expected to be 24 months. 20months for subject recruitment and 4 months for final subject follow-up.