
eg, codeine, hydrocodone, oxycodone, methadone, buprenorphine, tramadol, and fentanyl. Individual patients may have genetic variants of the enzymes, or may be taking inducer or inhibitor drugs, that strongly influence opioid metabolism and, hence, their detectable presence or absence in urine (or blood, or oral fluid if used) [see, Carlozzi et al. 2008; Smith 2009].