. Author manuscript; available in PMC: 2016 Dec 6.

Published in final edited form as: Clin Geriatr Med. 2016 Aug 9;32(4):725–735. doi: 10.1016/j.cger.2016.06.006

Table 2.

Hepatic metabolism pathways for opioids.^19,30

Phase	Isoenzyme	Substrate	Active Metabolite
I	CYP2D6	Codeine^a (prodrug)	Morphine
		Hydrocodone^a (prodrug)	Hydromorphone
		Tramadol (prodrug)	O-desmethyltramadol

	CYP3A4	Buprenorphine	None
		Fentanyl	None
		Meperidine	Normeperidine
		Methadone^a	None
		Oxycodone	Oxymorphone and noroxycodone

II	UGT	Hydromorphone	Hydromorphone-3-glucuronide
		Levorphanol^a	None
		Morphine	Morphine-3-and 6 glucuronide
		Oxymorphone^a	6 hydroxy-oxymorphone
		Tapentadol	Tapentadol O-glucuronide

Pharmacokinetics have not been studied in the elderly.

Abbreviation: UGT = uridine diphosphate glucuronosyltransferase.