Why do people vomit with a tonic clonic seizure?

Toxic effect. A seizure is a reaction of the brain to a toxic condition. If the region of the brain that controls nausea and vomiting are involved, then the seizure will cause vomiting. In addition, some patients develop migraine symptoms following a seizure. The vomiting can lead to aspiration pneumonia, that is why they should be laid on their side and nothing put in the mouth.