Diphenhydramine is an example of a first-generation antihistamine. (See Figures 5.7^[1] and 5.8.^[2]) Second-generation antihistamines were developed to have fewer side effects. An example of a second-generation antihistamine is cetirizine.

Mechanism of Action: Antihistamines have the following mechanisms of action: block histamine at H1 receptors; inhibit smooth muscle constriction in blood vessels and the respiratory and GI tracts; and decrease capillary permeability, salivation, and tear formation.

Indications: Antihistamines are used for relief of allergy or cold symptoms.

Nursing Considerations: This medication is not safe for children under the age of 2 years without a health care provider’s order.

Side Effects/Adverse Effects: First-generation medications can cause anticholinergic effects (such as dry mouth, urinary retention, constipation and blurred vision). CNS depression or CNS stimulation with excessive doses can occur, especially in children. Therefore, first-generation antihistamines should be used with caution in the elderly.

Second-generation medications may cause headache, nausea, vomiting, dysmenorrhea, and fatigue.^[3]

Patient Teaching & Education: Patients should be advised that antihistamines may cause drowsiness, and concurrent use of alcohol or other CNS depressants should be avoided. Patients should take only the recommended amount of medication and not to exceed dosing recommendations. Some patients may experience side effects such as dry mouth, and frequent oral hygiene may assist in alleviating discomfort.^[4]

Now let’s take a closer look at the medication grid for diphenhydramine and cetirizine in Table 5.6.^[5],[6],[7] Medication grids are intended to assist students to learn key points about each medication class. Basic information related to a common generic medication in this class is outlined, including administration considerations, therapeutic effects, and side effects/adverse effects. Prototype/generic medication listed in the medication grid is also linked to a free resource from the U.S. National Library of Medicine called DailyMed. Because information about medication is constantly changing, nurses should always consult evidence-based resources to review current recommendations before administering specific medication.

Table 5.6 Diphenhydramine and Cetirizine Medication Grid