I think he ate some toxic/poisonous candy.
Check the difference in your dictionary between 'poisonous', and 'poisoned'.
Then you tell me why these three sentences are correct:
1. In the fairy story, Snow White ate a poisoned apple
2. When one gathers wild mushrooms in the forest, it can be difficult to know which ones might be poisonous.
The Acme Chemical Company poured its waste by-products into the river, and the water became toxic to marine life.
Small amounts of the drug are beneficial for his heart condition, but in large quantities, it would prove toxic.
Student or Learner