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Antihistamines

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By Marianne Belleza, R.N.

Antihistamines selectively block the effects of histamine at the histamine-1 receptor sites, decreasing the allergic response. Antihistamines are used for the relief of symptoms associated with seasonal and perennial allergic rhinitis, allergic conjunctivitis, uncomplicated urticaria, and angioedema.

Learn about the uses and nursing care plan considerations and nursing diagnoses needed for patients taking antihistamines in this nursing pharmacology study guide.

Table of Contents

Antihistamines: Generic and Brand Names

Here is a table of commonly encountered antihistamines, their generic names, and brand names:

  • First-generation antihistamines
    • brompheniramine (Bidhist)
    • carbinoxamine (Histex, Palgic)
    • chlorpheniramine (Aller-Chlor)
    • clemastine (Tavist)
    • cyclizine (Marezine)
    • cyproheptadine
    • dexchlorpheniramine
    • dimenhydrinate (Dimetabs)
    • diphenhydramine (Benadryl)
    • hydroxyzine (Vistaril)
    • meclizine (Antivert)
    • promethazine (Phenergan)
    • triprolidine (Zymine)
  • Second-generation antihistamines
    • azelastine (Asteline)
    • cetirizine (Zyrtect)
    • desloratadine (Clarinex)
    • fexofenadine (Allegra)
    • levocetirizine (Xyzal)
    • loratadine (Claritin)

Disease Spotlight: Seasonal Rhinitis

Seasonal rhinitis is an inflammation of the nasal cavity, commonly called hay fever, that afflicts many people.

  • This condition occurs when the upper airways respond to a specific antigen (e.g., pollen, mold, dust) with a vigorous inflammatory response, resulting again in nasal congestion, sneezing, stuffiness, and watery eyes.

What are Antihistamines?

Antihistamines block the release or action of histamine, a chemical released during inflammation that increases secretions and narrows airways.

  • Antihistamines are found in multiple OTC preparations that are designed to relieve respiratory symptoms and treat allergies.
  • When choosing an antihistamine, the individual patient’s reaction to the drug is usually the governing factor.
  • Because first-generation antihistamines have greater anticholinergic effects with resultant drowsiness, a person who needs to be alert should be given one of the second-generation, less sedating antihistamines.
  • Because of their OTC availability, these drugs are often misused to treat colds and influenza.

Therapeutic Actions

The desired actions of antihistamines are as follows:

  • Selectively block the effects of histamine at the histamine-1 receptor sites, decreasing the allergic response.
  • Anticholinergic (atropine-like) and antipruritic effects.

Indication of Antihistamines

Antihistamines are indicated for the following:

  • Relief of symptoms of seasonal and perennial allergic rhinitis, allergic conjunctivitis, uncomplicated urticarial, and angioedema.
  • Relief of nasal and non-nasal symptoms of seasonal and perennial allergic rhinitis.
  • Relief of discomfort associated with dermographism; used as adjunctive therapy in anaphylactic reactions.
  • Relief of nausea and vomiting associated with motion sickness.

Pharmacokinetics of Antihistamines

The antihistamines are well-absorbed orally, with an onset of action ranging from 1 to 3 hours.

RouteOnsetPeakOnset
Oral15-30 min1-4h4-7h
IM20-30 min1-4h4-7h
IVRapid30-60 min4-8h
Half-life (T1/2)MetabolizationExcretion
2.5 to 7 hoursliverurine

Contraindications and Cautions

The following are contraindications and cautions when using antihistamines:

  • Pregnancy and lactation. Antihistamines are contraindicated during pregnancy and lactation unless the benefit to the mother clearly outweighs the potential risk to the fetus or baby.
  • Renal or hepatic impairment. They should be used with caution in renal or hepatic impairment, which could alter the metabolism and excretion of the drug.
  • Arrhythmias. Special care should be taken when these drugs are used by any patient with a history of arrhythmias or prolonged QT intervals because fatal cardiac arrhythmias have been associated with the use of certain antihistamines and drugs that increase QT intervals, including erythromycin.

Adverse Effects of Antihistamines

Adverse effects from the use of antihistamines include:

  • CNS: Drowsiness and sedation.
  • GI: Drying of the GI mucous membranes, GI upset, nausea.
  • GU: Dysuria, urinary hesitancy.
  • Skin: Skin eruption and itching.

Interactions

Interactions involved in the use of antihistamines include:

  • Monoamine inhibitor (MAOI). Anticholinergic effects may be prolonged if diphenhydramine is taken with a monoamine inhibitor.
  • Ketoconazole/erythromycin. Interaction of fexofenadine with ketoconazole or erythromycin may raise fexofenadine concentrations to toxic levels.

Nursing Considerations for Antihistamines

Nursing considerations for a patient using antihistamines include the following:

Nursing Assessment

History taking and examination of a patient using antihistamines may include the following:

  • Assess for possible contraindications or cautions: any history of allergy to antihistamines; pregnancy and lactation; and prolonged QT interval, which are contraindications to the use of the drug; and renal or hepatic impairment, which requires cautious use of the drug.
  • Perform a physical examination to establish baseline data for assessing the effectiveness of the drug and the occurrence of any adverse effects associated with the drug therapy.
  • Assess the skin color, texture, and lesions to monitor for anticholinergic effects or allergy.
  • Evaluate orientation, affect, and reflexes to monitor for changes due to CNS effects.
  • Assess respirations and adventitious sounds to monitor drug effects.
  • Evaluate renal and liver function tests to monitor for factors that could affect the metabolism or excretion of the drug.

Nursing Diagnosis and Care Planning

Nursing diagnoses and nursing care plans related to medical therapy with an antihistamine may include:

  • Acute pain related to GI, CNS or skin effects of the drug.
  • Disturbed sensory perception (Kinesthetic) related to CNS effects.
  • Deficient knowledge regarding drug therapy.

Nursing Implementation with Rationale

Nursing interventions for patients using antihistamines include the following:

  • Proper administration. Administer drug on an empty stomach, 1 hour before or 2 hours after meals, to increase the absorption.
  • Drug effectiveness. Note that the patient may have a poor response to one of these agents but a very effective response to another; the prescriber may need to try several different agents to find the one that is most effective.
  • Relief from dry mouth. Because of the drying nature of antihistamines, patients often experience dry mouth, which may lead to nausea and anorexia; suggest sugarless candies or lozenges to relieve some of the discomforts.
  • Safety measures. Provide safety measures as appropriate if CNS effects occur to prevent patient injury.
  • Increase fluid intake. Increase humidity and push fluids to decrease the problem of thickened secretions and dry nasal mucosa.
  • Ensure voiding. Have patient void before each dose to decrease urinary retention if this is a problem.
  • Skin care. Provide skin care as needed if skin dryness and lesions become a problem to prevent skin breakdown.
  • Avoid alcohol. Caution the patient to avoid alcohol while taking these drugs because serious sedation can occur.
  • Avoid OTC drugs. Caution the patient to avoid excessive dose and to check OTC drugs for the presence of antihistamines, which are found in many OTC preparations and could cause toxicity.
  • Health teaching. Provide thorough patient teaching, including the drug name and prescribed dosage measure to help avoid adverse effects, warning signs that may indicate problems, and the need for periodic monitoring and evaluation, to enhance patient knowledge about the drug therapy and promote compliance.
  • Encourage patient support. Offer support and encouragement to help the patient cope with the disease and the drug regimen.

Evaluation

Evaluation of a patient using antihistamines include:

  • Monitor patient response to the drug (relief of the symptoms of allergic rhinitis).
  • Monitor for adverse effects (skin dryness, GI upset, sedation and drowsiness, urinary retention, thickened secretions, glaucoma).
  • Evaluate the effectiveness of the teaching plan (patient can name drug, dosage, adverse effects to watch for, specific measures to avoid them, and measures to take to increase the effectiveness of the drug.
  • Monitor the effectiveness of comfort and safety measures and compliance with the regimen.

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References and Sources

The following are the recommended supplemental reading for this antihistamine nursing pharmacology study guide:

  • Roth, T., Roehrs, T., Koshorek, G., Sicklesteel, J., & Zorick, F. (1987). Sedative effects of antihistamines. Journal of Allergy and Clinical Immunology80(1), 94-98.
  • Vallerand, A. H. (2018). Davis’s drug guide for nurses. FA Davis. [Link]
  • Woosley, R. L. (1996). Cardiac actions of antihistamines. Annual review of pharmacology and toxicology36(1), 233-252.
Marianne leads a double life, working as a staff nurse during the day and moonlighting as a writer for Nurseslabs at night. As an outpatient department nurse, she has honed her skills in delivering health education to her patients, making her a valuable resource and study guide writer for aspiring student nurses.

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