Motion Sickness Drugs
Many of the drugs used to prevent motion sickness are antihistamines (see Section 2, Chapter 18). They are occasionally prescribed but are also available over the counter. Motion sickness drugs are most likely to be effective if taken 30 to 60 minutes before a trip.
Motion sickness drugs often make a person drowsy and less alert. In fact, one motion sickness drug, the antihistamine diphenhydramine, is the active ingredient in most OTC sleep aids. Anyone who performs an activity that requires alertness or concentration, including driving, should not take a motion sickness drug. A motion sickness drug should not be taken with alcohol, sleep aids, tranquilizers, or other drugs that also cause drowsiness and decrease alertness.
Other side effects of antihistamines are less common (see Section 2, Chapter 18). Infants and very young children may become agitated and should not be given a motion sickness drug except under a doctor's supervision. Too high a dose in a young child could lead to hallucinations or even seizures, which may be fatal.
Older people, pregnant women, and breastfeeding women should consult a doctor before they take any drug that contains an antihistamine, as should people with angle-closure glaucoma, heart disease, constipation, or an enlarged prostate gland.
 See the table Motion Sickness Drugs: Precautions for Children.
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