Sedating antihistamines and hyoscine are used for the prevention of motion sickness. They are much less useful if taken as treatment after nausea and vomiting have begun, as they may be vomited up before they can be absorbed and even if kept down will be more slowly absorbed as gastric motility is decreased.

Antihistamines marketed for travel sickness are: cinnarizine and promethazine. 'First-generation' H1-antagonists have, to varying degrees, anti-allergic, antipruritic, antitussive, antimuscarinic, sedative and anti-emetic properties.

Cinnarizine and promethazine appear to have more pronounced anti-emetic activity and are used mainly or exclusively for this purpose. Their effectiveness is thought to be due to not only their antimuscarinic activity but also to blockade of both histamine H1- and dopamine D2-receptors in the brain. Cinnarizine is a piperazine derivative and an effective anti-emetic. It causes some drowsiness, but antimuscarinic side-effects do not appear to be a problem.

Promethazine hydrochloride is a phenothiazine. It has marked anti-motion sickness activity, but also marked antimuscarinic properties and sedation is common. It is long-acting and a dose may be effective for up to 24 hours, so it may be useful for long journeys. The sedative effect is sometimes considered to be an advantage in young children on long journeys.

(Cyclizine is also licensed as a P medicine for prevention of motion sickness. However, it is not marketed for OTC sale and is subject to abuse for its euphoric effects.)

Hyoscine hydrobromide is a naturally occurring alkaloid that competitively inhibits the actions of acetylcholine at the muscarinic receptors of autonomic effector sites innervated by parasympathetic nerves. It has a central as well as a peripheral action, as it is lipid-soluble and crosses the blood€“brain barrier. It is probably the most effective drug for prevention of motion sickness,3 although it is relatively short-acting when used orally and has more pronounced antimuscarinic side-effects than antihistamines. Hyoscine is available as tablets and a skin patch, which can be left on and remains active for up to 72 hours.