This page of Baldurs Gate 3 game guide answers the question of how to get an antidote and cure a poisoned hero. Healing a controlled character may prove necessary after Nettie tricks and poisons the protagonist, imposing a powerful negative status on him.
As a reminder, you meet Nettie in Druid Grove as part of the Druid Apprentice quest. You can ask Nettie for help getting rid of the parasite. After you tell her the details of the hero's health condition, she'll invite you to the next room.
Nettie offers an antidote. If you pass the test successfully, you can determine before taking the cure that it is actually a poison. In such a situation, you can try to protect yourself peacefully from taking the poison by passing the persuasion test.
If you consume the poison, Nettie will inform you of this fact and explain why she acted this way. A strong negative status will be placed on the character - At Death's Door. It impedes all of the protagonist's abilities (main statistics). Moreover, this effect gets worse with each rest.
The antidote allows you to get the poison out of your system. You can acquire get it in several ways:
The first option is to get Nettie to give you the antidote by passing a skillcheck (e.g. persuasion, intimidation or deception).
The second option is to defeat Nettie in combat and then search her corpse - find the antidote.
The third option is to sneak up on Nettie and use the pickpocket option. If you pass the skill test correctly, you can take the antidote out of her pocket without being detected.
It is worth noting that Nettie, after poisoning the protagonist, will not let him leave the room by default. There is a solution to this problem - you can find and unlock the secret chamber shown in the image, which involves passing through poisonous fumes. The exit leads to an underground passage under the Druid Grove.