Antihistamines suppress the symptoms associated with histamine, but they do not alter the original cause of the reaction. They are used especially in the treatment of hay fever and certain other allergies. Antihistamines should be used with caution. Many cause drowsiness, and users are warned to avoid taking them before driving an automobile, operating dangerous machinery, or drinking alcohol.
Some tolerance usually develops over a week or two to the sedative action of antihistamines.
The newer antihistamines do not cause sleepiness and have therefore gained wide use. Antihistamines often cause dry mouth, may cause a fast heart rate, or may, unless taken with food, irritate the stomach.