Why Zyrtec (Cetirizine) is Taken at Night
Cetirizine is often taken at night primarily to allow any mild sedative effects (occurring in 13.7% of patients) to happen during sleep rather than interfering with daytime activities, though it can be taken at any time of day since it provides 24-hour symptom control. 1
The Sedation Profile of Cetirizine
Cetirizine causes mild drowsiness in approximately 13.7% of patients compared to 6.3% with placebo, making it more sedating than truly non-sedating alternatives like fexofenadine or loratadine. 1
The drug has low penetration of the blood-brain barrier and minimal central nervous system impairment compared to first-generation antihistamines, but it is not completely non-sedating at recommended doses. 1, 2
If sedation is desired or acceptable, bedtime dosing is advantageous, allowing any mild sedative effects to occur during sleep. 1
When Nighttime Dosing Makes Sense
For patients experiencing bothersome daytime sedation with morning dosing, switching to bedtime administration can improve daytime function while maintaining 24-hour allergy control. 1
Older adults are more sensitive to antihistamine effects and require extra caution—nighttime dosing may be particularly beneficial in this population to minimize daytime impairment. 1
Important Caveats About Nighttime Antihistamine Strategies
The common practice of adding a sedating first-generation antihistamine (like hydroxyzine) at night to a second-generation antihistamine by day is not supported by evidence and actually increases daytime somnolence without improving symptom control. 3
A randomized controlled trial demonstrated that levocetirizine monotherapy significantly reduced daytime somnolence compared to baseline, while adding hydroxyzine at night did not reduce daytime somnolence and provided no additional benefit for urticaria symptoms or sleep quality. 3
First-generation antihistamines dosed at bedtime still cause significant daytime drowsiness and performance impairment due to prolonged plasma half-lives. 4
Practical Dosing Recommendations
Cetirizine provides once-daily dosing with a rapid onset and long duration of activity, making timing flexible based on individual tolerance. 5, 6
Avoid taking more than one 10 mg dose in 24 hours, as higher doses increase sedation risk without necessarily improving efficacy. 1
Patients should be counseled about the possibility of mild drowsiness, and if this becomes problematic even with nighttime dosing, consider switching to fexofenadine, which is completely non-sedating. 1
Special Populations
In patients with moderate renal impairment (creatinine clearance 10-20 mL/min), the cetirizine dose should be halved, and it should be avoided entirely in severe renal impairment (creatinine clearance <10 mL/min). 7, 1
Paradoxically, one case report documented insomnia in a 23-month-old child taking cetirizine at bedtime, which resolved upon discontinuation—highlighting that individual responses can vary. 8