Antihistamines have been available for more than 50 years, and include older, first-generation versions, such as Benadryl (diphenhydramine), Atarax (hydroxyzine), Periactin (cyproheptadine) and Silenor (doxepin). These medications have significant side effects, and therefore led to the development of second-generation antihistamines, which include Claritin (loratadine), Zyrtec (cetirizine), and Allegra (fexofenadine), which have fewer side effects.
First-generation antihistamines have a significant amount of anticholinergic side effects. These side effects may include drowsiness, dry mouth, urinary retention (especially in men with prostate enlargement) and constipation. Due to the side effects of these medications, they are generally not recommended for routine, daily use. Since first-generation antihistamines can impair mental and motor functioning, they can impair a person’s ability to operate motor vehicles or heavy machinery. What's more, in many states in the U.S., a person can be charged with a DUI (driving under the influence) if an automobile accident is deemed to be their fault while taking a first-generation antihistamine.
In addition to sedation and dry mouth, antihistamines may have the side effect of increasing appetite and weight gain. This may be due to the similar chemical structure of antihistamines and certain psychiatric medications, such as antidepressants, which are known to increase appetite and lead to weight gain. Certain antihistamines, specifically Periactin, have actually been used for the purpose of increasing appetite and weight gain in underweight children and cancer patients undergoing chemotherapy.
DuBuske LM. Clinical Comparison of Histamine H1-Receptor Antagonist Drugs. J Allergy Clin Immunol. 1996;98:S307-18.
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