Phenylpiperidines

4-phenylpiperidines: the aryl group is attached directly to the piperidine ring, which is what separates them from the fentanyls, where an anilide nitrogen sits between the two. Pethidine is the reference member, with ketobemidone, alphaprodine and phenoperidine as relatives.

Two things in this family behave unlike the rest of the subject. Pethidine is metabolised to a compound that accumulates with repeated dosing and in kidney impairment and is convulsant rather than sedating, so naloxone does not treat that half and can unmask it; pethidine taken with an MAOI is a documented and potentially fatal interaction. Loperamide and diphenoxylate are antidiarrhoeals designed to stay in the gut, and at the amounts taken to force a central effect they block cardiac conduction — an opioid picture with a cardiac cause that opioid reversal does not fix.