I personally do not feel high on opiates and I tried most of them, I might be in the minority here.
It wouldn't surprise me if either were true - it's definitely the case that different people need different opiate doses to get the same effects (even for their first dose, not considering developed tolerance), I've known people for whom OTC (over the counter, no prescription needed) cocodamol - 8mg codeine + 500mg paracetamol, x1-2 per dose - contained enough codeine for them to feel a bit high, meanwhile other people like me need prescription strength (30-60mg) codeine to feel any benefits at all, and don't feel remotely high from it.
I also had a friend once who literally couldn't get high from weed, it has no effect on him - at least up to the amount tested, which was him smoking & vaping several grams of a strong strain of cannabis that got the rest of the room extremely high with much less of it. He tried on various other occasions, included edibles from a coffeeshop in Amsterdam, never any effects.
Most substances do not affect me the way they affect others; I have depression & anxiety and I have tried almost all antidepressants (along with mood stabilizers) to no effect.
I suspect it may have to do with my brain lesions (I have MS), but I am not quite sure.
That said, opiates do help with my physical symptoms, and only them, unfortunately.
> people like me need prescription strength (30-60mg) codeine to feel any benefits at all, and don't feel remotely high from it.
It is interesting though. Oxycodone is way stronger than codeine, and 80 mg is considered a very high dose yet it does not make me high, with or without tolerance.
My sister has defective P450 and your experience is the same as hers with opiates and other drugs, e.g. they are entirely ineffective when taken orally.