Dimenhydrinate and diphenhydramine for motion sickness should be taken at least 30 minutes before travel and are most effective when taken one to two hours beforehand. These drugs are for symptomatic relief, and a missed dose does not have any harmful consequences except for possible return of the symptoms. Too frequent dosing may cause increased sedation and other side effects. Side Effects and Risks Common side effects such as drowsiness may decrease somewhat as your body adjusts to the medicine. Some of these may be due to overdose, others to individual intolerance to the medication.
Most antihistamines cause some people to become drowsy.
Make sure you know how you react to the drug you are taking before you drive or operate machinery. The elderly may be particularly susceptible to the sedative effect. Do not give antihistamines to children under 6 years without consulting a doctor. Antihistamines add to the effects of alcohol and other depressants. Therefore, avoid taking them close to the same time you drink alcohol or use drugs that slow down the nervous system, such as sedatives, tranquilizers, sleeping pills, narcotics, prescription pain medicines, seizure medications, muscle relaxants, or anesthetics.