Edit: Would it be absurd to consider 6d10 of force damage, originating in the gut of a PC, as described below from the PHB to be immediately fatal? As-written it does a fair amount of damage (avg. 33) in a (ten foot radius?) area around the point of mixture.
I'm reading through the DMG in preparation for a meeting tomorrow with my group and, seeing as they are all newly minted level 4 "heroes", I've decided it is time for some magic items to enter the fray. The BBEG they are likely going to enter service with is going to allow them access to a portion of his armory and there will undoubtedly be scrolls and potions involved.
Reading the Variant: Mixing Potions got me worried, as a 01 roll of a d100 results in:
01: The mixture creates a magical explosion, dealing 6d10 force damage to the mixer and 1d10 force damage to each creature within 5 feet of the mixer.
This is all well and good, but there is this line above the table:
A character might drink one potion while still under the effects of another,
So if a PC decides to drink two potions, and you roll a 01 on the miscibility roll, wouldn't that effectively kill the PC? I don't care if you have the intestinal fortitude of an igneous rock, really, a forceful explosion (unless the damage roll is extremely low?) will wreck your internal organs wouldn't it?
Obviously the chances of this sort of thing happening are minute at best, but I can't imagine something exploding inside your chest cavity as anything but catastrophic, even with a low roll…
Best Answer
HP are an Abstraction
First, remember that HP are not just a pure measure of how much physical injury a creature can take before being reduced to chunky salsa. It is an abstraction of the creature's ability to avoid death. If a Fighter hits a Goblin for 20% of its total HP, that doesn't mean the Goblin's left leg was cut off or disabled. It means the Goblin twisted out of the way, taking a light cut to its arm and tweaking its leg. It is hurt and distracted, but not sufficiently injured to impede its ability to fight.
Similarly, the 6d10 force damage for a botched potion mix does not necessarily equate sitting on a brick of C4 as it goes off. Even though the damage is typed 'force' for the purposes of resistances and weakness, some of the actual 'damage' might be the headache, fuzzy vision, and ringing ears from the explosion. So the reason taking the explosion inside still does the same damage is because the abstraction shifts. Even though the PC is hurt more by the explosion inside, they don't see or hear the explosion so no fuzzy vision or ringing ears.
Does it even Work?
Arguably, mixing two potions in your stomach would not even cause this. It is not just the mixing of the ingredients that causes this, but the magic they contain. When you drink a potion, the magic is immediately imparted on you. You do not have to digest the potion first. So the magic is gone and you are left with a non-poisonous beverage. So by the time you drink the second potion, the two magics aren't mixing and there is no more harm than having any two or three or seven buff spells cast on you at the same time.
You might counter, what if the person drinks both potions at the same time? RAW says they can't. Even if a class ability or feat allows them to drink two potions in a single action, presumably they are still being chugged quickly but separately. In fact, if the class ability/feat does not specifically say this is an option, that would tend to be a strong indicator it is not.
What's good for the goose...
If you decide to rule that drinking two potions together results in a mixing scenario, and that the 6d10 damage result is instant death for the imbiber, be ready for the players to use it against you. Watch as they pour dozens of potions down a sleeping dragon's throat until one pair mixes badly, instantly killing it. Marvel as they slip potions into a wine tasting event to gorily kill the king and his court.