24.1 Antihistamines and Decongestants

Antihistamines and Decongestants

Antihistamines and decongestants are the frontline drugs for managing allergies and nasal congestion. Understanding how they work, their side effect profiles, and key nursing considerations will help you administer them safely and educate patients effectively.

Key Characteristics and Mechanisms of Action

Antihistamines block H1 histamine receptors throughout the body, which reduces histamine-mediated symptoms like sneezing, itching, and runny nose. The two generations differ mainly in how well they cross the blood-brain barrier:

• First-generation antihistamines (diphenhydramine, chlorpheniramine) readily cross the blood-brain barrier, which is why they cause significant sedation and stronger anticholinergic effects. They also have a shorter duration of action, typically requiring dosing every 4–6 hours.
• Second-generation antihistamines (loratadine, cetirizine, fexofenadine) are designed to have minimal blood-brain barrier penetration. This means much less sedation, making them preferred for daytime use. They also last longer, usually dosed once daily.

Decongestants stimulate alpha-1 adrenergic receptors on blood vessels in the nasal mucosa, causing vasoconstriction. This shrinks swollen tissue and opens the nasal passages. They come in two forms:

• Oral decongestants (pseudoephedrine, phenylephrine) are absorbed systemically, so they can raise blood pressure and heart rate. Pseudoephedrine is more effective but is kept behind the pharmacy counter due to its potential for methamphetamine production.
• Topical decongestants (oxymetazoline, xylometazoline) act directly on nasal tissue with fewer systemic effects. However, they carry a significant risk of rebound congestion (rhinitis medicamentosa) if used beyond 3–5 days.

Indications, Side Effects, and Drug Interactions

Indications include allergic rhinitis, hay fever, upper respiratory allergies, and common cold symptoms such as sneezing, runny nose, and nasal congestion.

Antihistamine side effects:

• Sedation and drowsiness (far more common with first-generation)
• Anticholinergic effects: dry mouth, dry eyes, dry nasal passages, constipation, urinary retention, blurred vision
• Older adults are especially vulnerable to anticholinergic effects and fall risk from sedation

Decongestant side effects:

• Increased blood pressure and heart rate
• Insomnia, anxiety, restlessness
• Rebound congestion with prolonged topical use (beyond 3–5 days)

Key drug interactions to know:

Critical Nursing Considerations

Assessment:

• Evaluate the patient's symptoms, allergy history, and current medications before administration
• Check for contraindications: decongestants are generally contraindicated in uncontrolled hypertension, severe coronary artery disease, and patients on MAOIs
• Obtain baseline vital signs, especially blood pressure and heart rate, before giving decongestants

Administration:

• Follow prescribed dosage and route
• For topical decongestants, teach the patient proper nasal spray technique: clear nostrils first, keep the head upright, spray while gently inhaling
• Advise patients taking first-generation antihistamines to avoid alcohol and other CNS depressants

Monitoring:

• Assess whether symptoms are actually improving after starting the medication
• Watch for adverse effects: excessive sedation with antihistamines, elevated blood pressure or tachycardia with decongestants
• For older adults on first-generation antihistamines, monitor closely for confusion, falls, and urinary retention
• Encourage patients to report any unusual symptoms promptly

Patient Education Plan

Proper use:

1. Take the medication at the prescribed dose and frequency. Do not double up on missed doses.
2. Limit topical decongestant use to 3–5 consecutive days maximum to avoid rebound congestion.
3. Do not exceed the recommended daily dose of oral decongestants.

Precautions:

• Avoid alcohol and sedating medications while taking first-generation antihistamines
• Use caution when driving or operating machinery, especially with first-generation antihistamines
• Patients with hypertension, heart disease, diabetes, or thyroid disorders should consult their provider before using any decongestant

Non-pharmacological measures:

• Minimize allergen exposure by keeping windows closed during high pollen counts, using air filters, and washing bedding frequently
• Nasal saline irrigation can help relieve congestion without medication
• Stay well hydrated and get adequate rest to support recovery

Follow-up:

• Schedule appointments to evaluate whether the treatment plan is working
• Contact the healthcare provider if symptoms worsen, persist beyond expected duration, or if new symptoms develop