The mechanism by which some antihistamines exert their antiemetic, anti–motion sickness, and antivertigo effects is not precisely known but may be related to their central anticholinergic actions. They diminish vestibular stimulation and depress labyrinthine function. An action on the medullary chemoreceptive trigger zone may also be involved in the antiemetic effect. Dimenhydrinate is a competitive antagonist at the histamine H1 receptor, which is widely distributed in the human brain. Dimenhydrinate's anti-emetic effect is probably due to H1 antagonism in the vestibular system in the brain.
Dimenhydrinate is an antiemetics drug combination that contains diphenhydramine and theophylline. It is not effective in the treatment of nausea associated with cancer chemotherapy. Dimenhydrinate directly inhibits the stimulation of certain nerves in the brain and inner ear to suppress nausea, vomiting, dizziness, and vertigo. Diphenhydramine and dimenhydinate both reduce vestibular neuronal excitation due to angular or linear acceleration motions.
Symptoms of overdose include delerium, hallucinations, and excitment. Patients may be violent and confused.