PH2.6 | PH2.6 | Antihistamines for Allergic Rhinitis — SDL Guide — Summary & Reflection

KEY TAKEAWAYS

Key takeaways from this module:

1. Histamine H1 receptors mediate the symptoms of allergic rhinitis (sneezing, itching, rhinorrhoea, conjunctivitis). H2 receptors on parietal cells regulate acid secretion — not allergy.
2. First-generation antihistamines (chlorpheniramine, diphenhydramine, promethazine, hydroxyzine, cyproheptadine) are lipophilic, cross the BBB readily, cause sedation, and have significant anticholinergic effects. They require multiple daily dosing.
3. Second-generation antihistamines (cetirizine, loratadine, fexofenadine, levocetirizine, desloratadine) minimally penetrate the BBB, are non-sedating, lack anticholinergic effects, and are the standard of care for outpatient allergic rhinitis.
4. Fexofenadine + fruit juice: grapefruit/apple/orange juices reduce fexofenadine bioavailability significantly — always take with water.
5. Allergic rhinitis management: mild intermittent → oral 2nd-gen antihistamine; moderate–severe persistent → intranasal corticosteroid ± antihistamine. Intranasal decongestants are limited to ≤5 days to avoid rhinitis medicamentosa.
6. Common cold: supportive management only; antihistamines are not evidence-based. Antibiotics are contraindicated.
7. Avoid 1st-gen agents in: drivers, elderly (fall risk, cognitive impairment), BPH (urinary retention), narrow-angle glaucoma, children under 2.
8. Historical 2nd-gen agents (terfenadine, astemizole) were withdrawn for QT prolongation — reinforcing the importance of pharmacovigilance.

REFLECT

Recall the patient from the opening case: the college student who was drowsy on chlorpheniramine. Her drowsiness occurred because chlorpheniramine crosses the blood-brain barrier and occupies central H1 receptors, producing sedation. A second-generation antihistamine — cetirizine, loratadine, or fexofenadine — would give her the same anti-allergic effect without impairing her studies.

Now consider a deeper question: why do patients and families still prefer chlorpheniramine-containing cold syrups? They are cheap, widely available, and the sedation is sometimes perceived as rest. As the prescribing clinician, your role is not just to choose the correct drug but to counsel patients on why the 'better medicine' is indeed better — and when the older drugs are still appropriate (e.g., when sedation is desired at bedtime in an itchy urticarial patient). Evidence-based prescribing is both a scientific and a communication skill.