24.1: Antihistamines and Decongestants

Learning Objectives

By the end of this section, you should be able to:

• 24.1.1 Identify the characteristics of antihistamine and decongestant drugs used to treat respiratory disorders.
• 24.1.2 Explain the indications, actions, adverse reactions, and interactions of antihistamine and decongestant drugs used to treat respiratory disorders.
• 24.1.3 Describe nursing implications of antihistamine and decongestant drugs used to treat respiratory disorders.
• 24.1.4 Explain the client education related to antihistamine and decongestant drugs used to treat respiratory disorders.

Antihistamines

Histamines are chemicals in the immune system. Histamines are released by the body as part of the immune response to a foreign substance such as an allergen or pathogen. Histamine receptors are activated, and reactions occur. H1 receptors are found in smooth muscle cells, endothelial cells, and nerve endings. Activation of these receptors leads to various responses, including smooth muscle contraction, increased vascular permeability, itching, and sensory nerve stimulation. These receptors are involved in allergic reactions as well as regulating sleep–wake cycles and maintaining blood pressure. H2 receptors are mostly located in the stomach parietal cells, regulating gastric acid secretion. When these receptors are blocked, allergy symptoms can be reduced and stomach acid can be controlled.

Once the immune system detects an allergen, a sequence of events is triggered to safeguard the body. Chemical messages are dispatched to mast cells and basophils in various body parts, signaling them to release histamines (see Figure 24.2). These histamines then augment blood circulation in the vicinity of the allergen, which leads to inflammation. To illustrate, if the nose is impacted, histamines stimulate the mucous membranes to produce additional mucus, resulting in nasal congestion, sneezing, and runny nose. These symptoms can typically be relieved using antihistamines (Fowler, 2022).

Table 24.1 Drug Emphasis Table: First-Generation Antihistamines (source: https://dailymed.nlm.nih.gov/dailymed/)

Adverse Effects and Contraindications

First-generation antihistamines can cause drowsiness, dizziness, confusion, and urinary retention. Clients with a history of urinary retention or prostatic hyperplasia should use first-generation antihistamines cautiously. First-generation antihistamines should be avoided in older adults because they can increase the risk of dementia.

Table 24.2 is a drug prototype table for first-generation antihistamines featuring diphenhydramine. It lists drug class, mechanism of action, adult dosage, indications, therapeutic effects, drug and food interactions, adverse effects, and contraindications.

Table 24.2 Drug Prototype Table: Diphenhydramine (source: https://dailymed.nlm.nih.gov/dailymed/)

Drug Class Antihistamine Mechanism of Action Antagonizes histamine H1 receptor sites, thereby hindering the actions triggered by histamine	Drug Dosage 25–50 mg orally every 4–6 hours, not to exceed 12 capsules in 24 hours.
Indications Rhinitis Allergy symptoms Motion sickness Nighttime sleep aid Therapeutic Effects Reduces nasal congestion, sneezing, and coughing Causes drowsiness that can aid in sleeping Prevents motion-related nausea	Drug Interactions Other central nervous system (CNS) depressants MAO inhibitors Food Interactions Alcohol
Adverse Effects Drowsiness Dizziness Confusion Hypotension Dry mouth Urinary retention	Contraindications Hypersensitivity Caution: Angle-closure glaucoma Stenosing peptic ulcer Prostatic hyperplasia

Safety Alert

Antihistamines and Older Clients

First-generation antihistamines should be avoided in older clients. Their highly anticholinergic properties can increase the risk of dementia, and clearance of the drug is reduced in clients of advanced age.

Second-Generation Antihistamines

The mechanism of action and side effects of second-generation antihistamines are distinct from those of first-generation antihistamines. Unlike first-generation antihistamines, which block both histaminic and muscarinic receptors and can penetrate the blood–brain barrier, second-generation antihistamines primarily target histamine receptors and do not cross the blood–brain barrier, which therefore diminishes the likelihood of central nervous system–related adverse effects. This distinction leads to a significantly reduced occurrence of first-generation side effects, especially drowsiness, in second-generation antihistamines (Naqvi & Gerriets, 2023).

Cetirizine

Initially, cetirizine was a prescription-only second-generation antihistamine, but it was subsequently approved as an over-the-counter (OTC) medication and is now readily accessible. It is a potent treatment for allergic rhinitis and effectively alleviates symptoms such as sneezing, watery eyes, and rhinorrhea. Additionally, cetirizine has been identified as a primary therapeutic option for individuals with chronic urticaria. The effectiveness of cetirizine and its minimal side effects make it a very popular option for clients (Naqvi & Gerriets, 2023).

Levocetirizine

Levocetirizine is another second-generation antihistamine that effectively treats allergic rhinitis and its associated symptoms. Like other second-generation antihistamines, including cetirizine, it does not penetrate the blood–brain barrier, resulting in a lower occurrence of side effects when compared to first-generation antihistamines (DailyMed, Levocetirizine, 2011).

Loratadine

Loratadine is also a second-generation antihistamine and is available over the counter. It is effective in pruritus, allergic rhinitis, and sneezing related to seasonal allergies. Loratadine should not be used if a client is pregnant or breastfeeding until they have consulted with their health care provider (MedlinePlus, 2022).

Table 24.3 lists common second-generation antihistamines and typical routes and dosing for adult clients.

Table 24.3 Drug Emphasis Table: Second-Generation Antihistamines (source: https://dailymed.nlm.nih.gov/dailymed/)

Drug	Routes and Dosage Ranges
Cetirizine (Zyrtec)	One 10 mg tablet orally once daily. Do not take more than one 10 mg tablet daily. A 5 mg product may be used for less severe symptoms and for older clients.
Levocetirizine (Xyzal)	One 5 mg tablet orally once daily in the evening. Do not take more than one 5 mg tablet in 24 hours. One-half of a tablet (2.5 mg) once daily may be appropriate for less severe symptoms.
Loratadine (Claritin)	One 10 mg tablet orally daily, not to exceed more than 1 tablet in a 24-hour period. Also available in chewable and dissolving tablets and oral solution.

Adverse Effects and Contraindications

Second-generation antihistamines have a lower incidence of adverse effects than first generation, but they can still cause drowsiness, sedation, dry mouth, and insomnia. Clients with hepatic or renal impairment should use second-generation antihistamines cautiously, and those with hypersensitivity to the medication should avoid use.

Table 24.4 is a drug prototype table for second-generation antihistamines featuring loratadine. It lists drug class, mechanism of action, adult dosage, indications, therapeutic effects, drug and food interactions, adverse effects, and contraindications.

Table 24.4 Drug Prototype Table: Loratadine (source: https://dailymed.nlm.nih.gov/dailymed/)

Drug Class Antihistamine, second generation Mechanism of Action Blocks effects of histamine at H1 receptor sites	Drug Dosage One 10 mg tablet orally daily, not to exceed more than 1 tablet in a 24-hour period. Also available in chewable and dissolving tablets and oral solution.
Indications Allergic rhinitis To relieve itching due to hives (urticaria) Therapeutic Effects Relieves runny nose, sneezing, and itchy watery eyes caused by allergies Relieves itching due to hives	Drug Interactions Ketoconazole Macrolide antibiotics (may increase loratadine level) Food Interactions Alcohol
Adverse Effects Headache Drowsiness Sedation Fatigue Insomnia Nervousness Dry mouth	Contraindications Hypersensitivity Caution: Hepatic or renal impairment

Third-Generation Antihistamines

The latest type of antihistamines, known as third-generation antihistamines, are among the newest antihistamines available. They were designed to effectively treat the same conditions as previous antihistamines but without the potential risk of cardiac toxicity that was observed in clients with metabolic issues. This new generation offers a safer alternative for such individuals, and as with the second generation, the likelihood of experiencing negative side effects is decreased. The reduced incidence of side effects is related to the inability of third-generation antihistamines to cross the blood–brain barrier and affect the central nervous system. Second- and third-generation antihistamines are favored compared to the first generation due to their effectiveness and safety profile (Ricciardi et al., 2019).

Fexofenadine is a recently developed third-generation antihistamine (Sakur et al., 2022) for managing allergic rhinitis and chronic urticaria, and it has been approved for use by individuals of all ages, including children. Compared to other antihistamines, fexofenadine has demonstrated greater efficacy in relieving eye and nasal congestion symptoms. Clients who are pregnant or breastfeeding should seek advice from their health care provider before taking this medication because it can decrease breast milk production. It is not usually prescribed during pregnancy. Fexofenadine can cause headaches and dizziness (NHS, 2021).

Desloratadine, an active metabolite of the second-generation antihistamine loratadine, is a highly selective medication that is considerably more powerful than loratadine in treating allergy symptoms and urticaria (DailyMed, Loratadine, 2009).

Table 24.5 lists common third-generation antihistamines and typical routes and dosing for adult clients.

Table 24.5 Drug Emphasis Table: Third-Generation Antihistamines (source: https://dailymed.nlm.nih.gov/dailymed/)

Drug	Routes and Dosage Ranges
Fexofenadine (Allegra)	12-hour tablet: 60 mg orally with water every 12 hours. No more than 2 tablets in 24 hours. 24-hour tablet: 180 mg orally with water once daily. No more than 1 tablet in 24 hours.
Desloratadine (Clarinex)	5 mg tablet orally once daily; if using the syrup, 2 teaspoonfuls (5 mg in 10 mL) orally once daily.

Adverse Effects and Contraindications

Third-generation antihistamines, much like second-generation antihistamines, have a lower incidence of adverse effects than first generation, but they can still cause fatigue, dizziness, nausea, and insomnia. Third-generation antihistamines should be used cautiously in clients with impaired renal function, and those with hypersensitivity should avoid using them.

Table 24.6 is a drug prototype table for third-generation antihistamines featuring fexofenadine. It lists drug class, mechanism of action, adult dosage, indications, therapeutic effects, drug and food interactions, adverse effects, and contraindications.

Table 24.6 Drug Prototype Table: Fexofenadine (source: https://dailymed.nlm.nih.gov/dailymed/)

Drug Class Antihistamine, third generation Mechanism of Action Inhibits peripheral H1 receptors	Drug Dosage 12-hour tablet: 60 mg orally with water every 12 hours. No more than 2 tablets in 24 hours. 24-hour tablet: 180 mg orally with water once daily. No more than 1 tablet in 24 hours.
Indications Seasonal allergies Chronic idiopathic urticaria Therapeutic Effects Relief of allergy symptoms such as runny nose and itchy, watery eyes Relief of itching from chronic urticaria	Drug Interactions Aluminum or magnesium antacids Erythromycin Ketoconazole Rifampin Food Interactions Fruit juices (may decrease drug effects) Alcohol
Adverse Effects Fatigue Fever Headache Dizziness Insomnia Otitis media Rhinorrhea Nausea Abdominal pain	Contraindications Hypersensitivity Caution: Impaired renal function

Safety Alert

Antihistamines

Ingesting antihistamines in doses exceeding the recommended amount, particularly diphenhydramine, may result in severe heart complications, seizures, unconsciousness, or even fatality. Clients should avoid combining antihistamines with substances that are also sedating (sedatives, alcohol, etc.).

Clinical Tip

Antihistamine Education

When educating clients on the use of antihistamines, the nurse should tell them to call the health care provider if their symptoms last for more than 1 week, worsen, or are accompanied by other symptoms such as a headache that will not go away, a fever, or a rash. These may indicate a more serious medical condition.

Special Considerations

Antihistamines

Antihistamines can cause marked drowsiness and should be avoided when driving or operating machinery.

Alcohol and other CNS depressants can increase drowsiness and should be avoided when taking antihistamines.

Clients also should tell their health care provider if they are pregnant, plan to become pregnant, or are breastfeeding.

(Source: MedlinePlus, 2022)

Decongestants

Decongestants refer to medications that offer temporary relief for a blocked nose or nasal congestion. They can alleviate the stuffiness commonly associated with allergies, sinusitis, colds, and the flu. These medications function by diminishing the inflammation of the blood vessels in the nose, resulting in opening of the air passage. Two types of decongestants are systemic decongestants and topical, or nasal, decongestants (Corren, 2017).

Nasal Decongestants

Nasal decongestants are typically available as drops or sprays applied directly to the nasal passage. These remedies are regarded as topical agents and do not have any widespread impacts on the body. They are usually favored over oral drugs since they deliver prompt relief from nasal congestion. Nasal decongestants should be used for 3 days or less, as rebound congestion can occur. Rebound congestion is worsened congestion after the medication wears off.

Oxymetazoline

Oxymetazoline is a nasal spray that helps alleviate nasal congestion induced by allergies and colds. It may also relieve sinus pressure and congestion. Despite its convenience, this medication should be used only briefly. Overuse, beyond 3 days, can result in rebound congestion, where inflammation of the nasal mucosa occurs due to excessive use of nasal drops or sprays. Oxymetazoline is considered a local decongestant because it acts on the nasal tissue by narrowing the blood vessels and opening the air passages to allow for ease of breathing, producing no systemic effects. The lack of systemic effects can be beneficial to clients with hypertension or other cardiac disorders.

Adverse Effects and Contraindications

Rebound congestion can occur if used for more than 3 days. Some clients may experience temporary discomfort such as burning when used. Those who are hypersensitive should avoid their use.

Table 24.7 is a drug prototype table for the nasal decongestant oxymetazoline. It lists drug class, mechanism of action, adult dosage, indications, therapeutic effects, drug and food interactions, adverse effects, and contraindications.

Table 24.7 Drug Prototype Table: Oxymetazoline (source: https://dailymed.nlm.nih.gov/dailymed/)

Drug Class Nasal decongestant Mechanism of Action Causes vasoconstriction of the nasal mucosa to alleviate nasal congestion	Drug Dosage 2–3 sprays in each nostril not more often than every 10–12 hours. Do not exceed 2 doses in any 12-hour period. Do not use for longer than 3 days to avoid rebound congestion.
Indications Nasal congestion caused by common colds, allergies, and other respiratory conditions Therapeutic Effects Reduces nasal congestion	Drug Interactions Albuterol Bupropion Food Interactions Caffeine
Adverse Effects Temporary discomfort such as burning or stinging Rebound congestion (if used for more than 3 days)	Contraindications Hypersensitivity

Systemic Decongestants

Systemic decongestants act by causing vasoconstriction of the nasal mucosa, relieving nasal congestion. Unlike topical nasal decongestants, systemic decongestants can cause more side effects. Nervousness, heart palpitations, insomnia, and tachycardia can be seen. Systemic decongestants act on alpha-adrenergic receptors in the blood vessel walls and can cause an increase in blood pressure and affect the cardiovascular system. Clients with cardiac issues should use systemic decongestants with caution because they can also increase blood pressure.

Pseudoephedrine

Pseudoephedrine is a systemic decongestant used to relieve nasal congestion. Pseudoephedrine can be purchased over the counter; however, it is highly regulated due to the use of some components of the drug to make methamphetamine. The drug typically must be requested from the pharmacist, but specific regulations may vary by state (American Addiction Centers, 2023). Clients should avoid caffeine when taking this drug, and those with blood pressure issues should discuss use of this medication with their health care provider. Pseudoephedrine is often found in combination with other medications. Clients should check labels carefully to avoid taking too much of the same medication (MedlinePlus, 2018).

Phenylephrine

Phenylephrine is another common nasal decongestant. This drug is an alpha-adrenergic agonist. It can be administered via tablet, liquid, or dissolving strip. It works as other decongestants do by reducing the swelling of the blood vessels in the nose. As with pseudoephedrine, those with blood pressure issues should consult with their health care provider before taking this medication because it can cause an increase in blood pressure.

Table 24.8 lists common systemic decongestants and typical routes and dosing for adult clients.

Table 24.8 Drug Emphasis Table: Systemic Decongestants (sources: https://dailymed.nlm.nih.gov/dailymed/; MedlinePlus, 2018)

Drug	Routes and Dosage Ranges
Pseudoephedrine (Sudafed)	One 120 mg tablet orally every 12 hours; do not take more than 2 tablets in 24 hours. One 240 mg tablet orally every 24 hours; do not take more than 1 tablet in 24 hours. Immediate release: 30–60 mg orally every 4–6 hours; maximum dose: 240 mg/day.
Phenylephrine (Sudafed PE)	One 10 mg tablet orally every 4 hours; do not take more than 6 doses in 24 hours.

Adverse Effects and Contraindications

Systemic decongestants can cause increased blood pressure as well as nervousness, dizziness, and sleeplessness. Those clients with a hypersensitivity should avoid their use, and clients with heart disease and high blood pressure should use the drugs with caution and under the guidance of their health care provider.

Table 24.9 is a drug prototype table for systemic decongestants featuring pseudoephedrine. It lists drug class, mechanism of action, adult dosage, indications, therapeutic effects, drug and food interactions, adverse effects, and contraindications.

Table 24.9 Drug Prototype Table: Pseudoephedrine (source: https://dailymed.nlm.nih.gov/dailymed/)

Drug Class Systemic nasal decongestant Mechanism of Action Acts directly on adrenergic receptors and produces vasoconstriction, which shrinks nasal mucosa	Drug Dosage One 120 mg tablet orally every 12 hours; do not take more than 2 tablets in 24 hours. One 240 mg tablet orally every 24 hours; do not take more than 1 tablet in 24 hours. Immediate release: 30–60 mg orally every 4–6 hours; maximum dose: 240 mg/day.
Indications Relief of nasal congestion caused by colds, flu, and allergies Therapeutic Effects Relief of nasal congestion	Drug Interactions Metoprolol Albuterol Metformin Guaifenesin Monoamine oxidase inhibitors (MAOIs) Food Interactions Caffeine Alcohol
Adverse Effects Nervousness Dizziness Insomnia	Contraindications Hypersensitivity Caution: Heart disease High blood pressure Diabetes

Link to Learning

Adrenergic Agonists

Access multimedia content

This video discusses how decongestants work to alleviate nasal decongestion.

Safety Alert

Phenylephrine vs. Pseudoephedrine

For clients with high blood pressure, phenylephrine should be used instead of pseudoephedrine. Phenylephrine has less impact on blood pressure than pseudoephedrine.

Clinical Tip

Decongestant Education

When educating clients on the use of decongestants, the nurse should tell them to call their health care provider if their symptoms last for more than 1 week, worsen, or are accompanied by other symptoms such as a headache that will not go away, a fever, or a rash. These may indicate a more serious medical condition.

Special Considerations

Decongestants

Decongestants and nasal drops should be used per the label instructions and not more to avoid rebound congestion.

Decongestants are not safe for children under age 6.

Clients should increase fluid intake when taking any cold medication, including decongestants.

(Source: MyHealth.Alberta.ca Network, 2022)

Nursing Implications

The nurse should do the following for clients who are taking antihistamines or decongestants:

• Prior to administering, assess the client’s medical history, current drug list, and allergies for potential interactions and contraindications.
• Educate the client on adverse effects of antihistamines including drowsiness, dizziness, and dry mouth.
• Educate the client on adverse effects of decongestants including nervousness, dizziness, and rebound congestion (nasal sprays).
• Monitor vital signs for hypotension and respiratory depression (antihistamines) and hypertension and irregular heart rhythms (decongestants).
• Monitor urinary input and output for urinary retention.
• Initiate fall precautions for older clients due to the adverse effects of hypotension, dizziness, and drowsiness (antihistamines).
• Provide oral care and lozenges or saliva substitute for dry mouth (antihistamines).
• Instruct the client on administration technique for nasal decongestant spray.
• Provide client teaching regarding the drug and when to call the health care provider. See below for client teaching guidelines.

Client Teaching Guidelines

The client taking an antihistamine should:

• Check with their health care provider before taking if they have glaucoma, peptic ulcer disease, or urinary retention or are pregnant.
• Take antihistamines only as prescribed, not exceeding the recommended amount.
• Use sugarless candies or lozenges if dry mouth is experienced.

The client taking an antihistamine should not:

• Drive a car, operate machinery, or perform other tasks that require alertness when taking first-generation antihistamines due to their causing drowsiness and/or dizziness.
• Take more than one antihistamine at a time. Often OTC drugs contain a combination of drugs that may contain antihistamines.

The client taking a decongestant should:

• Take the drug as directed.
• Report any adverse effects or symptoms not relieved within 1 week.
• Be sure to take in fluids and maintain hydration.
• Use a humidifier to prevent drying of the nasal passages.
• Avoid exposure to triggers that may cause congestion.

The client taking a decongestant should not:

• Use nasal sprays for more than 3 days.
• Consume caffeine and alcohol while taking decongestants.
• Take any OTC drugs with this medication without consulting their health care provider.
• Take the drug within 2 hours of bedtime because it may cause restlessness and insomnia.