CYP2B6 is The most crucial CYP enzyme associated with the metabolism of S-methadone and to an extent, R-methadone. The expression of CYP2B6Opioids (Mixed Agonist / Antagonist): May well diminish the analgesic effect of Opioid Agonists. Management: Search for alternatives to combined agonist/antagonist opioids in patients getting pure opioid agonist