OPRM1 and respiratory depression: In addition, dihydrocodeine has several active metabolites (DHC-6-O-glucuronide, dihydromorphine, dihydromorphine-3-O-glucuronide, dihydromorphine-6-O-glucuronide, and nordihydrocodeine), all of which are active at the mu opioid receptor responsible for mediating respiratory depression [12].