Pharmacologic Profile

General Use

Relief of symptoms associated with allergies, including rhinitis, urticaria, and angioedema, and as adjunctive therapy in anaphylactic reactions. Some antihistamines are used to treat motion sickness (dimenhydrinate and meclizine), insomnia (diphenhydramine), and other nonallergic conditions.

General Action and Information

Antihistamines block the effects of histamine at the H₁ receptor. They do not block histamine release, antibody production, or antigen-antibody reactions. The 1st-generation antihistamines (diphenhydramine) have anticholinergic properties and may cause constipation, dry eyes, dry mouth, and blurred vision; in addition, these antihistamines cause sedation (because they cross the blood-brain barrier). The 2nd-generation antihistamines (cetirizine, fexofenadine, levocetirizine, loratadine) do not cross the blood-brain barrier and are much less likely to cause sedation.

Contraindications

Hypersensitivity and angle-closure glaucoma. Should not be used in premature or newborn infants.

Precautions

Older adults may be more susceptible to adverse anticholinergic effects of antihistamines. Use cautiously in patients with pyloric obstruction, prostatic hypertrophy, hyperthyroidism, cardiovascular disease, or severe liver disease. Use cautiously in pregnancy and lactation.

Interactions

Additive sedation when used with other CNS depressants, including alcohol, antidepressants, opioid analgesics, and sedative/hypnotics. MAO inhibitors prolong and intensify the anticholinergic properties of antihistamines.

Nursing Implications

Assessment

• Assess allergy symptoms (rhinitis, conjunctivitis, hives) before and periodically throughout therapy.
• Assess lung sounds and character of bronchial secretions. Maintain fluid intake of 1500–2000 mL/day to ↓ viscosity of secretions.
• Nausea and Vomiting: Assess degree of nausea and frequency and amount of emesis when administering for nausea and vomiting.
• Pruritus: Observe the character, location, and size of affected area when administering for pruritic skin conditions.

Implementation

• When used for prophylaxis of motion sickness, administer ≥30 min and preferably 1–2 hr before exposure to conditions that may precipitate motion sickness.

Patient/Family Teaching

• Inform patient that drowsiness may occur. Avoid driving or other activities requiring alertness until response to drug is known.
• Caution patient to avoid using concurrent alcohol or CNS depressants.
• Advise patient that good oral hygiene, frequent rinsing of mouth with water, and sugarless gum or candy may help relieve dryness of mouth.
• Instruct patient to contact health care professional if symptoms persist.

Evaluation/Desired Outcomes

• Decrease in allergic symptoms.
• Prevention or decreased severity of nausea and vomiting.
• Relief of pruritus.
• Sedation when used as a hypnotic.