Gas Exchange

5.6 Antihistamines


Diphenhydramine and Cetirizine are two commonly-seen antihistamine drugs. In this chapter we explore Diphenhydramine.

Diphenhydramine is an example of a first-generation antihistamine. (See Figures 5.6a[1] and 5.6b.[2])  Second-generation antihistamines were developed to have fewer side effects. An example of a second-generation antihistamine is cetirizine.

An opened package of Benadryl.
Figure 5.6a Diphenhydramine is a first-generation antihistamine that is available orally or as an IV medication
A bottle of diphenhydramine HCI.
Figure 5.6b Diphenhydramine HCl preparation, single dose vial for IV administration

Mechanism of Action

Antihistamines have the following mechanisms of action: blocks histamine at H1 receptors; inhibits smooth muscle constriction in blood vessels and the respiratory and GI tracts; and decreases capillary permeability, salivation, and tear formation.

Indications for Use

Antihistamines are used for relief of allergy or cold symptoms.

Nursing Considerations Across the Lifespan

This medication is not safe for children under the age of 2 years without a healthcare provider’s order.

Adverse/Side 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]

Client Teaching & Education

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

Diphenhydramine Medication Card

Now let’s take a closer look at the medication card for diphenhydramine and cetirizine in Table 5.6.[5], [6], [7] Medication cards 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 card is also hyperlinked to a free resource from  Daily Med.  Because information about medication is constantly changing, nurses should always consult evidence-based resources to review current recommendations before administering specific medication.

Medication Card 5.6.1: Diphenhydramine

Class: Antihistamines (first generation)

Prototypes: Diphenhydramine (Benadryl)

Mechanism: Blocks histamine at H1 receptor, inhibits smooth muscle constriction in blood vessels and respiratory and GI tracts: decreases capillary permeability, salivation and tear formation

Therapeutic Effects

  • Relieves allergic reactions and temporarily relieves symptoms due to hay fever or other upper respiratory allergies: runny nose; sneezing; itchy, watery eyes; itching of the nose or throat. For example: Common cold symptom management


  • PO/IV/IM/Suppository
  • Common dosages for adult PO 25-50mg q4-6hrs PRN. IV 10-50mg IM or IV
  • Common dosages for children 4 years and older 1.25mg per kg or body weight injected IM (intramuscularly) up to 4 times per day


  • Common Cold
  • Respiratory Allergies
  • Mild allergic reactions


  • Avoid alcohol and CNS depressants due to risk of sedation

Side Effects

  • Sedation Anticholinergic effects Gastrointestinal: Nausea/Vomiting
  • Paradoxical effect: excitation in children
  • SAFETY: Note the name is close to Dimenhydramine (gravol) so double check you have the right medication

Nursing Considerations

  • Administer as per policies. Sedation is a very serious consideration especially with intravenous administration.
  • Monitor respirations ensure resps are over 12 resps per minute
  • Consider second and third generation antihistamines since they have less sedative properties

  1. "Benadryl Allergy USA" by ZenBenjamin is licensed under CC BY-NC-SA 2.0
  2. "diphenhydramine (1)" by Intropin is licensed under CC BY-NC 2.0
  3. Frandsen, G. & Pennington, S. (2018). Abrams’ clinical drug: Rationales for nursing practice (11th ed.). Wolters Kluwer.
  4. uCentral from Unbound Medicine.
  5. This work is a derivative of Pharmacology Notes: Nursing Implications for Clinical Practice by Gloria Velarde licensed under CC BY-NC-SA 4.0.
  6. Frandsen, G. & Pennington, S. (2018). Abrams’ clinical drug: Rationales for nursing practice (11th ed.). Wolters Kluwer.
  7. This work is a derivative of Daily Med by U.S. National Library of Medicine in the public domain.


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Fundamentals of Nursing Pharmacology - 1st Canadian Edition Copyright © 2023 by Chippewa Valley Technical College; Amanda Egert; Kimberly Lee; and Manu Gill is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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