There may be causes as follows.
Chronic pharyngitis can cause nausea and dry vomiting due to irritation of the throat by the smell of toothbrush or toothpaste.
Patients with oral diseases such as gingivitis, pericoronitis and calculus can also cause varying degrees of nausea and dry vomiting when brushing their teeth
People with digestive system disorders in part cause a conditioned reflex when brushing their teeth, which can produce nausea and dryness.