If a character drinks part of a potion, what happens? I assume it depends on the potion...or does it? Is alchemy divisible?
E.g.:
You can't drink part of a potion—That is, you can't drink enough to enjoy any of its effects without automatically chugging the whole thing.
Page 139 of the Dungeon Master's Guide states:
"Once used, a potion takes effect immediately, and it is used up."
One can read this in either of two ways:
Quaffing the potion takes no time (from a rules perspective). As soon as you've pressed the bottle to your lips, the potion is inside you all of a sudden, with no chance to drink only part of it.
You can drink whatever amount you want of the potion at any time, but it hasn't been used until you drink all of it—or drink enough that no one else can use it. Up until the potion qualifies as "used," anyone can drink it, but nobody gains its benefits.
By either interpretation, potion usage is an all-or-nothing proposition. Dungeon Masters can houserule the effects of partial potions, but there's really no support for it in the Fifth Edition text.
However, there's a sense in which you can "drink part of a potion" as much as you want: DMG 136, discussing the identification of potions, says "a little taste is enough to tell the taster what the potion does." So there's a benefit to taking a little sip, but it's not really what you're looking for.
In 5e, I have not seen any such provision, but there is precedent in earlier versions. As far back as the original DMG in AD&D 1e, it says this about potions:
Unless otherwise stated, the effects of a potion will last for 4 complete turns, plus 1-4 additional turns (d4). If half a potion is quaffed, the effects will last one-half as long in some cases.
Note that at the time, a "turn" was 10 minutes, and a "round" was 1 minute. With our current 6-second rounds, the equivalent would be for a potion to last 5-8 minutes, or half that for half a potion. Most of the 5e potion descriptions, OTOH, give a duration for a full potion of an hour.
A few potions were done a bit differently, for example:
Extra-Healing: This potion restores 6-27 (3d8+3) hit points of damage when wholly consumed, or 1-8 hit points of damage for each one-third potion.
In 5e, I have ruled that a standard healing potion (2d4+2) is too small to divide up, but that the stronger healing potions can be subdivided into an equivalent number of doses of standard healing potions.
With as rare as magic items are in 5e, I'm inclined to not allow dividing them into subdoses for shorter duration. "Here, everyone take a sip of this Frost Giant Strength potion, and we'll all be 23 Str for a few minutes, and let's go kick the bleep out of that dragon." = Recipe for Disaster
