Crafting and Harvesting Poison

During downtime between adventures, a character can use the crafting rules in the Player’s Handbook to create basic poison if the character has proficiency with a poisoner’s kit. It is possible to craft other kinds of poison. Not all poison ingredients are available for purchase, and tracking down certain ingredients might form the basis of an entire adventure.

A character can instead attempt to harvest poison from a poisonous creature, such as a snake, wyvern, or carrion crawler. The creature must be incapacitated or dead, and the harvesting requires 1d6 minutes followed by a DC 20 Intelligence (Nature) check. (Proficiency with the poisoner’s kit applies to this check if the character doesn’t have proficiency in Nature.) On a successful check, the character harvests enough poison for a single dose. On a failed check, the character is unable to extract any poison. If the character fails the check by 5 or more, the character is subjected to the creature’s poison.

from DMG page 258