Antihistamines and hyoscine possess peripheral and central antimuscarinic activity and have similar adverse effects, including sedation, dry mouth, blurred vision, urinary retention and constipation. However, these do not normally cause problems at the low doses used over short periods.

All should be avoided by patients with glaucoma or prostatic hypertrophy, and in general they should be used with caution in elderly people and in patients with epilepsy or cardiac or cardiovascular disease.

Paradoxical CNS stimulation may occur with antihistamines in children, resulting in insomnia and excitement and, rarely, nightmares, hallucinations and even convulsions. Alcohol should be avoided by patients taking any preparation for motion sickness.

In pregnancy, use of antihistamines is only advised under medical supervision because of fears of possible congenital malformations. No warnings have been issued against the use of hyoscine in pregnancy. However, it is prudent to avoid any medication in pregnancy if possible, and certainly in the first trimester.