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), Parkinson-like reactions (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. Most antihistamines have anticholinergic properties and may cause constipation, dry eyes, dry mouth, and blurred vision. In addition, many antihistamines cause sedation. Some phenothiazines have strong antihistaminic properties (hydroxyzine and promethazine).

Contraindications

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

Precautions

Elderly patients 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.
• Monitor pulse and BP before initiating and throughout IV therapy.
• Assess lung sounds and character of bronchial secretions. Maintain fluid intake of 1500–2000 mL/day to decrease viscosity of secretions.
• Assess degree of nausea and frequency and amount of emesis when administering for nausea and vomiting.
• Assess mental status, mood, and behavior when administering for anxiety.
• Observe the character, location, and size of affected area when administering for pruritic skin conditions.

Potential Nursing Diagnoses

• Ineffective airway clearance (Indications)
• Risk for injury (Adverse Reactions)
• Deficient knowledge related to disease processes and medication regimen (Patient/Family Teaching)

Implementation

• When used for prophylaxis of motion sickness, administer at least 30 min and preferably 1–2 hr before exposure to conditions that may precipitate motion sickness.
• When administering concurrently with opioid analgesics (hydroxyzine, promethazine), supervise ambulation closely to prevent injury secondary to increased sedation.

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.