I had minor surgery last year and was prescribed codeine (Tylenol 3) for pain. I found (as I had a couple times before when I took codeine for pain) that it wasn t any more effective than just the Tylenol. I know there are people who lack an enzyme that converts codeine to morphine. but in grad school i had a severe back spasm and was given morphine (or so I was told) in an oral tablet. It was also ineffective. The doctor was surprised. So anyway, this just got me thinking, are there people for whom morphine is just not effective? Is there a test? I d think this is something that should be in my chart for future reference.