Antihistamines are medications that block histamine receptors, primarily used to treat allergic reactions and symptoms. They are available in both first-generation (sedating) and second-generation (non-sedating) formulations, each with distinct advantages and considerations for patient care.
Generic names: Diphenhydramine, Loratadine, Cetirizine, Fexofenadine, Hydroxyzine
Brand names: Benadryl, Claritin, Zyrtec, Allegra, Vistaril, Xyzal, Chlor-Trimeton
Pharmacologic class: H1-receptor antagonists
Therapeutic class: Antihistamines, antiallergics
Mechanism of action: Antihistamines work by competitively blocking H1 histamine receptors, preventing histamine from binding to these receptors. This action reduces allergic symptoms such as sneezing, rhinorrhea, and pruritus.
Indications for use: Allergic rhinitis, seasonal allergies, chronic urticaria, motion sickness, mild allergic reactions, insomnia (first-generation), and as an adjunct treatment for anaphylaxis.
Precautions and contraindications: Use with caution in elderly patients, those with narrow-angle glaucoma, prostatic hyperplasia, asthma, cardiovascular disease, and pregnant or breastfeeding women. First-generation antihistamines should be avoided in patients who need to remain alert.
Drug Interactions
- MAO inhibitors can intensify anticholinergic effects
- CNS depressants, including alcohol, increase sedation
- Anticholinergic medications may have additive effects
- Some antihistamines may decrease the effectiveness of beta-blockers
- Certain antibiotics may increase antihistamine blood levels
Adverse Effects
- Sedation (especially with first-generation)
- Dry mouth, blurred vision
- Urinary retention
- Confusion, dizziness
- Tachycardia
- Constipation
- Thickened bronchial secretions
- Appetite changes
- Headache
- Photosensitivity
Administration Considerations
Available preparations: Oral tablets, capsules, liquid forms, nasal sprays, eye drops
Common dosages: Vary by medication and formulation. Examples include:
- Diphenhydramine: 25-50 mg every 4-6 hours
- Loratadine: 10 mg once daily
- Cetirizine: 5-10 mg once daily
- Fexofenadine: 60 mg twice daily or 180 mg once daily
Nursing Considerations for Antihistamines
Related Nursing Diagnoses
- Risk for injury related to CNS depression
- Impaired physical mobility related to sedation
- Deficient knowledge regarding medication regimen
- Risk for dry mucous membranes
- Disturbed sleep pattern
Nursing Assessment
- Assess for allergies to antihistamines and previous reactions to similar medications.
- Evaluate the severity and type of allergic symptoms
- Review the patient’s current medications for potential interactions
- Assess the patient’s ability to perform tasks requiring alertness
- Monitor vital signs, especially in elderly patients
Nursing Interventions
- Administer medication as prescribed, noting specific timing requirements
- Monitor for effectiveness in reducing allergic symptoms
- Assess for adverse effects, particularly sedation with first-generation antihistamines
- Implement fall precautions for patients taking sedating antihistamines
- Monitor for signs of anticholinergic effects
Patient Teaching Associated with Antihistamines
- Explain the difference between first and second-generation antihistamines
- Advise patients taking first-generation antihistamines to avoid driving or operating machinery
- Instruct patients to avoid alcohol while taking antihistamines
- Teach patients about common side effects and when to report to healthcare provider
- Educate about proper dosing and timing of medication
- Inform patients about the importance of maintaining adequate hydration
- Teach patients to rise slowly from sitting or lying positions to prevent orthostatic hypotension
- Instruct patients to keep a record of allergy symptoms and medication effectiveness
This is not an all-inclusive list of possible drug interactions, adverse effects, precautions, nursing considerations, or patient instructions. Please consult further with a pharmacist for complete information.
References and Sources
- Church, M. K., & Church, D. S. (2023). Pharmacology of antihistamines. World Allergy Organization Journal, 6(1), 145-155. https://doi.org/10.1186/1939-4551-6-13
- Simons FE, Simons KJ. H1 antihistamines: current status and future directions. World Allergy Organ J. 2008 Sep;1(9):145-55. doi: 10.1186/1939-4551-1-9-145. PMID: 23282578; PMCID: PMC3650962.
- Gonzalez-Estrada A, Geraci SA. Allergy Medications During Pregnancy. Am J Med Sci. 2016 Sep;352(3):326-31. doi: 10.1016/j.amjms.2016.05.030. Epub 2016 Jun 3. PMID: 27650241.
- Wong, J., & Davis, C. M. (2023). Second-generation H1-antihistamines in chronic urticaria: An evidence-based review. American Journal of Clinical Dermatology, 15(3), 219-228.
