The Dungeon Master's Guide (p. 258) describes a risk if you badly fail to harvest poison from a creature:
A character can … attempt to harvest poison from a poisonous creature … 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.) … If the character fails the check by 5 or more, the character is subjected to the creature's poison.
The Tool Proficiencies section of Xanathar's Guide offers advice on how to use tools, including the Handle Poison section of the Poisoner's Kit (p. 83) tool description:
Your proficiency allows you to handle and apply a poison without risk of exposing yourself to its effects.
Would this mean that a character who is proficient with the Poisoner's Kit could happily fail the harvest poison check by 5 or more without being subjected to the creature's poison?
Best Answer
Harvesting poison is not quite the same as handling and applying.
As you point out, failing the check by 5 or more will mean that the PC is subject to the effects of a creature's poison while trying to harvest.
Harvesting the poison may involve a tricky procedure of milking poison glands or removing a sting, etc. This is not that same as handling the actual poison itself (after harvesting). In the Dungeon Master's Guide it mentions harvesting poison from creatures such as a Wyvern or Purple Worm (p. 258) that are either dead or incapacitated. An incapacitated creature can't take actions or reactions, but it may not remain incapacitated for the full 1d6 minutes required to harvest the poison. So, that could potentially be another risk presented during harvesting.
At my table -by RAW- I would say they are subjected to the poison on a failed check by 5 or more. Personally, because the XGE was published later, I might provide a lower DC or give the player advantage on the roll.
Once harvested however, the PC would definitely not be affected by the poison when handling and applying it.
If your PC or a fellow player's PC has Protection from Poison, then they could cast this afterwards if the DC was failed - to neutralise the poison.