Even with healing potions, the people I play with don't tend to use more than one within a minute. I find that potions tend to be one of those things that the people I play with like to hoard. "Don't use that potion now because you might need it more later!" type of thing.
I think most of them won't even be aware of the potion miscibility rules, so it's not that they're avoiding using more than one potion so as not to risk the mixing effect but rather that potions are so precious that they don't want to use any unless they really have to, so the possibility of using multiple potions at once just never comes up!
So here's a funny story. We all knew that drinking multiple potions was bad in the day, but apparently nobody realized that there was a slim chance of a potion becoming permanent...until someone finally did.
Our local munchkin (as in, he thought of himself as a rules lawyer, but he was really just looking for exploits, and was infamous for failing to notice any fine print, and our DM rarely took him to task for this) apparently got super lucky in a battle.
He'd drunk a potion of giant strength and the tides of battle turned so he (either forgetting about miscibility or no longer caring because he was dead either way) downed a potion of extra-healing.
I wasn't there for this, mind you, I heard about it after the fact. The DM rolls some dice, asks the player "odds or evens"? Rolled another die and was like "ok".
Later, after the potion should have worn off, the player learned he now has
permanent giant strength! He quickly boasted about his windfall to the entire playgroup, and reveled in his newfound might, outclassing every other fighting character with sheer numbers.
This led to a sudden frenzy of players hoarding potions, all so they could attempt to replicate the same feat! A lot of mishaps occurred. This went on for a couple of months, and the best thing that ever happened was that someone got 6 extra hit points from a permanent healing potion until someone finally got the bright idea to cast
augury before imbibing!
For reasons that will soon be abundantly clear, this particular DM hated spells that could predict the future. He loved the randomness of D&D (he rolled dice for everything, like Gary Gygax on Futurama!) and the idea that he'd be sudden beholden to having fate decreed bugged him. Now for most uses of
augury, he would routinely answer "weal and woe" unless something was a really, really bad idea, because you know, maybe something bad could happen, but you might score some treasure, right? This caused the spell to fall out of favor, but every so often, a player would find a Boolean scenario and the DM would grit his teeth.
This all came to a head when we found a
deck of many things. The spell got it's first nerf that day, as the DM declared that, being a "minor artifact", the Deck was immune to the effects of mortal magic to divine it's effects.
So the spell was already on the chopping block. Attempting to use it with potion mixing was the last straw. The DM later declared that the Gods would stop granting the spell or answering it. Being a fairly decent DM, he tied it to goings-on in his campaign, but it was pretty easy to divine (pun intended!) the truth of the matter.
So maybe all you need to do is have the players see the results of positive potion mixing, and they'll start gambling the way my group did!
For extra fun, introduce a
bag of beans to the game as well!
