Herremann the Wise said:
Your DMing. The player says that they have three vials taped together. They swallow the collective contents. You as the DM say:
I, as the GM, say "What, are you TRYING to be a jerk?"
If they insisted that this is what they are going to do (never mind where they found tape in the campaign setting...), then maybe I tell them that the shape of the vials makes it hard to get the necks close enough together for simultaneous drinking, and so they spill two of them on their shirt and get to roll 1d3 to see which one they actually swallowed.
Or if they say they mix three potions in a cup and drink it? Perhaps I'd say okay, that'll take three rounds to uncap the vials, pour 'em into the cup, mix it, and drink it...and if you're not being a wanker about it, I'll probably just let all three potions work normally. If you're annoying me, then, gee, I guess the efficacy of those potions diminishes when they're exposed to the air for that long, and maybe none of them will work.
But if I wasn't in the mood to humor a bad idea, I'd probably just tell them that their idea won't work and that they should just drink one potion at a time like the rules say.
Mind you, I'm not obsessed with following the rules in every situation, but there has to be a good enough reason to make me want to bend them. This idea (chugging multiple potions simultaneously) just doesn't have any good reasons behind it. I'm not feeling especially inclined to throw out a perfectly acceptable rule stating that drinking a potion is a standard action which can invite an attack of opportunity just on a whim, and I'm certainly not interested in opening the door for characters to start taping together three
cure light wounds potions instead of paying for a single potion of
cure serious wounds (thereby saving as much as 600gp and only losing a few extra hit points' worth of curing, no less!).
At some point you really just need to be able to say "Sorry, but NO."
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good ideas can get the rules bent to fit them: dumb ideas never will