This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pharmacology
Which of the following drug is most commonly used world wide in maintenance doses for opioid dependence? 
Select and explain. 
A. naltrexone, B. Methadone, C. Imipramine, D. Disulfiram
B: Methadone
Answer: B: Methadone 
Explanation: Ans. is 'b' i.e., Methadone Methodone maintenance is the most common form of pharmacoligical treatment for opioid dependence.