Oh woah, sincere apologies! (not sleeping for the past 48hrs+ means my brain isn't as sharp as it usually is!). I'll try to explain it as much as I can without rambling (I do that! =P).
Basically between person to person (even say within the same family for instance) there's is a degree of bio-variability - differences in the way your physical make-up is brought together then say, your sister - from that fact that your hair might be a lighter colour to, in the example I said above, defects in parts of the body.
Once well known one is that 10% of the white population (about 6% of people from Arab decent, about 3% from black decent), purely due to random changes/mutations, mean that for those people, that enzyme (the specific enzyme is called CYP2D6) isn't able to metabolise as well as the other 90%. However even within this simple example, there lies variability where one person may just feel a little ill to someone else being knock unconscious!
I personally find this example interesting since this one (and many others in it's family) also metabolise anything from White Grapefruit Juice, to certain anti-depressants, to an the OP said codeine (and codeine analogues like morphine or oxycodone).
And this is just one example. Another one many people not know is the enzyme which breaks down alcohol (called alkoholdehydrogenase) is most often (again to varying degrees) defective in people from Asian decent. That's why, if you've every noticed, it doesn't take much for an Asian person to get drunk basically because their body just can't processes it as well at, say Westerners.
Did I explain myself? If you have any questions, PM me! I'd be happy to explain whatever I can
Oh, it may dry up your nose if it's been running recently too!
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