Antihistamines and hyoscine may potentiate the actions of other drugs that cause sedation or have antimuscarinic effects, including tricyclic antidepressants, monoamine oxidase inhibitors, phenothiazines, hypnotics, nefopam, amantadine and disopyramide.

Dry mouth caused by the antimuscarinic effects of antihistamines and hyoscine may reduce the effect of sublingual nitrates.

There appears to be little published evidence on the efficacy of motion sickness treatments available for OTC sale. Most studies relate to seasickness. Hyoscine is probably the most effective drug.¹ Some studies relating to cinnarizine have been published, with conflicting results.^2,3