there are two sets of two different genes that we all have that allow us to process opiates. the first set is used to convert the non-morphine type opiates into a morphine type opiate. if you are lacking one of these, you will not convert very well, or at a slower pace. because of this, those that are missing the first gene will get little to no effect from non-morphines, these same people will also experience nausea at times. if lacking both of the first 2 genes than these people are essentially incapable of taking the first class, this includes vicodins, norco, codeine, etc.
People that are lacking one gene from the second set, are unable to use the converted substance efficiently, thus they need large doses. I.E. non opiate tolerant individuals have been seen to take what would be a toxic dose of morphine in most people and receive almost no analgesic effect due to their ability to process the morphine effectivly. Also, these same people will have issues with serotonin as these same genes play in there as well.
If a person is lacking in both sets of genes, than they can have a wide array of issues that most users never encounter. Likely we have all encountered them, they just get nothing from opiates, or do not like what they get. there is no way to tell if you are one of these people. There are also people that possess extra copies of these genes, they also have varying reactions, likely they will be very sensitive or eliminate the drug immediately as these same genes play into elimination as well.
Good luck, be safe out there.
when you excercise a little self control, you can do your shit for ever.