One potential issue I can see (because this can happen with each of these three substances) is rate of ingestion. What happens if you get enough down to black out, but do so BEFORE you've invested your entire amounts? Being blacked out on these and doing things (like driving) certainly can and does happen. If you managed to have it all mixed in a small volume and tossed it all back in one go you might have a better chance of success...or, you could throw up enough of it that what's left in your system is dangerous (see driving), but no longer lethal.
Does that make sense? It does in my head but I'm not sure if my explanation came out right.