Antihistamines block the action of histamine, which normally is released in the body in

allergic reactions. Histamine causes allergic symptoms such as hives, bronchial asthma,

hay fever, and in severe cases, anaphylactic shock (dyspnea, hypotension, and loss of

consciousness). Antihistamines cannot cure the allergic reaction, but they relieve its

symptoms. Many antihistamines have strong antiemetic (prevention of nausea) activity and

are used to prevent motion sickness. The most common side effects of antihistamines are

drowsiness, blurred vision, tremors, digestive upset, and lack of motor coordination.

Common antihistamines are listed as following:

cetirizine (Zyrtec)

chlorpheniramine maleate (Chlor-Trimeton)

diphenhydramine (Benadryl)

fexofenadine (Allegra)

loratadine (Claritin)

meclizine (Antivert)

promethazine (Phenergan)