Most items under Adventuring Gear (pg 150 of PHB) either explicitly state that they require an action (the Use Object action, to be specific) to use (eg. Caltrops, Holy Water, Climber's Kit) or obviously don't require an action in an of themselves, but could be used in general roleplaying ways (eg. Block and Tackle, a Book, a Crowbar).
The 2 exceptions seem to be Healing potions and Antitoxins.
Healing Potions are covered in the DMG and fall under Use Magic Item (an action), but nothing is said about Antitoxins.
Does it take an action to drink an Antitoxin, or how can one drink one otherwise?
Best Answer
RAW, it appears to be usable as the free object interaction on your turn.
In the SRD, a Potion of Healing has the following description (emphasis mine):
Crawford confirms that drinking a Potion of Healing is the "Use Magic Item" action.
Conversely, this is the SRD description for Antitoxin:
It neither describes Antitoxin as magical, nor specifies that it requires an action. It also is not described as a potion, so the blanket potion rule in the DMG (drinking or administering a potion requires an action) doesn't appear to apply.
Antitoxin is also not listed in the magic item section of the DMG where every other RAW potion is included.
So, it appears that this is a free object interaction. The reason this is not explicitly forbidden is possibly because Antitoxin only grants advantage on saving throws against poison. Drinking multiple antitoxins has no effect, so there's no strong reason to explicitly limit it to one per turn as with other potions.