One of my characters is sort of a cougar-selkie. He has a cat skin that he needs to wear to change shape, but the actual catalyst of the change is some hard-to-explain magic bolognium in powder form. Because he doesn't like the physical sensation of changing shape, he cuts the magic some kind of opiate to numb the feeling. Because he's the kind of person who just doesn't think, he mixes the stuff with rum (or something else hard and fairly sweet)to kill the weird flavor. If I had to guess, I'd say his Magic Brew was probably around 1.5 parts morphine, three or four parts booze.
But leaving the magical aspect out of the equation, I just have to wonder: is he going to be remotely functional once that crap works its way into his system? Reading up confirmed my general impression that mixing alcohol and opiates is not a good idea, but nothing I've found really gets into amounts necessary to achieve certain results. He's a pretty big guy, around 6'1 and two hundred pounds. He isn't really taking it in huge doses, either. Probably an ounce or less at a time semi-frequently. I assume that it isn't enough for instantaneous death or coma, but is he going to have it together enough to behave normally?
And if is going to completely knock him out, would I be stretching my believability to fall back on "it's maaaagic!" as an explanation for why he's up and around and coherent?