It is most easily acquired before Hardmode, when the Underground Jungle is still relatively safe.
The Poisoned status effect can be exceptionally deadly early on in higher difficulties, especially Master Mode where it sometimes lasts over 40 seconds. The Bezoar takes up a valuable accessory slot that the player may not have to spare, though. Instead, you can simply keep the Bezoar in your inventory, and briefly put it on to cleanse yourself of the status effect before returning it to your inventory, allowing you to remove the poison without having to wear it all the time.