<prompt> Mechanism of action of methadone isOpioid receptor agonistOpioid receptor partial agonistOpioid receptor antagonistOpioid receptor inverse agonist  <response>: Ans. is a i.e., Opioid receptor agonist Methadone Methadone is opioid receptor agonist. Most important feature is high oral: parenteral activity ratio 1:2 and firm binding to tissue protein. It relieves cancer and neuropathic pain not responding to morphine. Abuse potential is less than morphine. It also blocks NMDA receptors and uptake of monoamines. It is the drug of choice for substitution therapy of opioid dependence. 1 mg of methadone can be substituted for 4 mg of morphine. Methadone is less hypnotic than morphine sedative action is less intense. 