Why should it even be an issue? How hard is it to pick one vice and run with it? For instance, if you are going to do GHB, then make it a GHB night and don't drink any alcohol... if you're going to drink, then don't bring any GHB into the picture... I would say it's best to give yourself at LEAST a couple hours between the two, but you're still playing with fire - don't get burnt!
pvc -
The alcohol your friend consumed on Friday had little to do with the overdose, as alcohol isn't in you're system for that long... sounds like he got some fucked up shit, or else his body is wack...
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"I am not one of those weak-spirited, sappy Americans who want to be liked by all the people around them. I don't care if people hate my guts; I assume most of them do. The important question is: 'What are they in a position to do about it?'" (William S. Burroughs)