Question

Why does John's head swim when he eats the food from the glass bottles

Answer 1

As John enters the "city of the Gods," he realizes that he is hungry, and so goes on the hunt for some food.  He states that it is a well-known fact that the gods didn't hunt for food, but rather got food from "enchanted boxes and jars," and that it was forbidden to eat any of that food, because it often made one sick.