OTC Antihistamines: Regular vs PRN Dosing
For seasonal and perennial allergic rhinitis, continuous daily use of OTC antihistamines is most effective, but PRN (as-needed) use is appropriate for episodic allergic rhinitis due to their relatively rapid onset of action. 1
Dosing Strategy Based on Rhinitis Type
Continuous Daily Dosing (Preferred for SAR/PAR)
- Continuous use provides superior symptom control for seasonal allergic rhinitis (SAR) and perennial allergic rhinitis (PAR) compared to intermittent dosing 1
- Regular daily dosing maintains consistent therapeutic blood levels and prevents symptom breakthrough 1
- Second-generation agents (loratadine, desloratadine, fexofenadine, cetirizine) are preferred for continuous use due to once-daily dosing and minimal sedation at recommended doses 1
PRN Dosing (Appropriate for Episodic Rhinitis)
- PRN use is clinically appropriate for episodic allergic rhinitis because oral antihistamines have relatively rapid onset of action 1
- This strategy works when allergen exposure is predictable and intermittent 1
- Patients can take medication before anticipated allergen exposure or at symptom onset 1
Clinical Considerations
Why Continuous Dosing Works Better
The guideline evidence clearly states that while antihistamines can be used PRN, continuous use is most effective for managing persistent allergic conditions 1. This is because:
- Antihistamines work primarily through competitive inhibition of histamine receptors, making them more effective at preventing rather than reversing histamine's actions 2
- Consistent receptor blockade prevents the inflammatory cascade from initiating 2
When PRN Dosing Is Acceptable
PRN dosing remains a reasonable option when 1:
- Symptoms are truly episodic (not daily or near-daily)
- Allergen exposure is predictable and avoidable most of the time
- Patient preference strongly favors intermittent use
- Cost or access issues limit daily medication use
Important Caveats
Avoid First-Generation Antihistamines
- Second-generation agents should be strongly preferred over first-generation antihistamines (diphenhydramine, hydroxyzine) to avoid sedation, performance impairment, and anticholinergic effects 1
- First-generation antihistamines cause sedation that is often subjectively unperceived but still impairs function 1
- The Canadian Society of Allergy and Clinical Immunology recommends newer generation antihistamines as first-line due to superior safety profiles 3
Dosing Frequency Matters
- Loratadine and other second-generation agents are dosed once daily, making adherence easier for continuous use 4
- First-generation agents like diphenhydramine require dosing every 4-6 hours, making them impractical for continuous prophylaxis 5
Alternative Considerations
If antihistamines alone (whether continuous or PRN) provide inadequate control, intranasal corticosteroids are the most effective monotherapy for SAR and PAR and should be considered 1