There could be many reasons why people vomit after drinking a gallon of milk in a short time like 30 minutes including lactose intolerance, or too much Calcium suddenly in the body from the milk that the body cannot handle.
But the real reason is the size of a human stomach and it can only hold about half a gallon (1.9 liters) of anything including milk at a time. When you drink more than that capacity, then, the stretch receptors present in the stomach gets stimulated and causes vomiting.
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