<qa>  What is a synthetic mu opioid agonist?Methadone is a synthetic mu opioid agonist that has been proved as clinically effective in pain management. However, methadone usage for pain control in Thailand has been limited because physicians are not familiar with its dosing and concern about the risk of drug accumulation and cardiac arrhythmia. Therefore, this prospective study was conducted to evaluate the efficacy and safety of a methadone protocol in Thai patients with severe chronic noncancer and cancer pain. 