The kind of thing I'm describing is somewhat similar to combining an MAOI with a drug that's metabolized through a pathway that the MAOI suppresses: if you took either drug on its own, it wouldn't be a problem, but when you combine them, it's dangerous. I previously thought that a CNS depressant was a CNS depressant, but it sounds like that's not entirely true, since some CNS depressants inhibit respiration and some do not.

Ah, but that sounds different from what other posters were saying, i.e., that combining certain CNS depressants is _worse_ than overdosing on either of them on their own, because they act on _different_ systems. So with GHB and alcohol, is that actually the case? Does one of them depress respiration and the other one does not? Or when people overdose on a combination of GHB and alcohol, is it just that they took a full dose of both and the combination was the same as it would have been if they had taken that big of an overdose of either one?

To put it another way, with certain combinations, it seems like the sum is greater than the whole of the parts (2+2=5), because Drug B is causing Drug A to create an effect that Drug A wouldn't create on its own, even if you overdosed on it.

That's really interesting, thanks! This might answer a long-time question I've had, which is why some drugs are worse in combination than a higher dose of them would be on their own. I had always wondered how combining GHB and alcohol could be any worse than doing double the normal dose of either one on its own, but it sounds like maybe this is why?