Page "Hydrocodone" Paragraph 34

Some of the effects of hydrocodone come from the fact that a fraction of it is changed to hydromorphone in the liver, as is the case with all codeine-based analgesics ( codeine into morphine, dihydrocodeine into dihydromorphine, nicocodeine into nicomorphine etc .).

The percentage can vary based on both other medications taken and inherited metabolic quirks involving the Cytochrome P450 metabolic pathways — some cannot process it at all, whereas a smaller percentage can get even more strength from it than usual.

These factors can also cause hydrocodone and related drugs to have a threshold effect, cause significant lengthening or shortening of the duration of effects in the absence of tolerance, and increase or decrease the de facto conversion ratio between hydrocodone and other drugs like morphine, hydromorphone, and synthetics like levorphanol and methadone.