The sedating antihistamines cinnarizine, promethazine hydrochloride, promethazine theoclate and hyoscine are the only drugs licensed for non-prescription sale, as P medicines, for travel sickness.

In addition to their anti-allergic, antipruritic, antitussive and antimuscarinic effects, first-generation H1 antagonists also have antiemetic properties in varying degrees, and some are used exclusively for these actions.

Their effectiveness may relate to their antimuscarinic activity, but it has also been proposed that the antiemetic activity results from blockade of dopamine D2 receptors in the brain.

The compounds licensed for motion sickness are selected primarily for their antiemetic properties, but factors such as duration of action and side effects are also taken into account.

All are intended for prophylactic use and are less effective once nausea or vomiting has begun; apart from the risk of the drugs being vomited up, they are absorbed more slowly because gastric motility is decreased.