Dosage!
Professor Sprout, the Hufflepuff Head of House, was usually warm and kind, with a gentle smile. But when she discussed this with her favorite apprentice, Hannah Abbott, her expression was grave.
All the substances people considered deadly poisons were, in the Magical World, simply things that exerted an influence from a certain angle. It was just that this influence could be overwhelmingly powerful when it came to an individual "human."
For example, the scream of a Mandrake could kill a person, but it was really just "a violent vibration that caused the soul to leave the body." If you adjusted the dosage properly, that same vibration could be used to treat many ailments.
Professor Sprout had once shown Hannah how to stroke a Mandrake's little ears with both hands, making it produce the right effect. In an instant, it could destroy the effects of a powerful Wizard's Full Body-Bind Curse.
It could even produce a sustained, widespread magical effect that prevented a Full Body-Bind Curse from covering an entire building.
If you interacted with Mandrakes just right, you could even treat problems such as Soul-Loss Syndrome, Chronic Insomnia, and Attention Deficit without making patients take any medicine—ailments that would otherwise require complicated treatment from a Healer.
So...
Could Anton really be fine after eating so many deadly poisons?
Absolutely not. No matter how powerful Anton's ability to use Magic was, he was still human. Cut him with a knife and he would bleed; stub his toe on a bedpost and he would howl in pain; miss out on sleep and he would yawn nonstop.
Hannah Abbott had Professor Sprout's patient guidance. Anton, too, had received careful instruction from Professor Snape.
For a Potions Master or Charms Master, daring to stuff herbs into their mouth and taste their effects came