Zyrtec (cetirizine) is taken once a day. The standard adult dose is 10 mg, and its effects last a full 24 hours, so there’s no need to take it more than once daily. The timing is flexible, but consistency matters: pick a time that works for your routine and stick with it.
Dosing by Age
Adults and children 12 and older take 10 mg once daily. For milder symptoms, 5 mg once daily may be enough. The FDA label notes that most adults in clinical trials used the 10 mg dose.
Children 6 to 11 take 5 mg or 10 mg once daily, depending on symptom severity. Children 2 to 5 start at 2.5 mg once daily. If that’s not enough, the dose can go up to 5 mg per day, either as a single dose or split into 2.5 mg every 12 hours. For children 6 months to under 2 years, the starting dose is 2.5 mg once daily, and children 12 to 23 months can increase to a maximum of 5 mg per day (split into two doses 12 hours apart).
Adults 65 and older should take no more than 5 mg in 24 hours, since the body clears the drug more slowly with age.
The 24-Hour Maximum
Do not exceed one dose per day. For adults, that ceiling is 10 mg in 24 hours. For young children aged 2 to 5, the absolute maximum is 5 mg in 24 hours. Taking more won’t improve symptom relief and increases the likelihood of drowsiness.
If you’re still symptomatic at the standard dose, that’s a signal to explore other approaches with your doctor rather than doubling up.
Morning vs. Night
Zyrtec provides 24-hour relief regardless of when you take it, so there’s no clinically superior time of day. Most people take it in the morning, which aligns well with daytime allergy exposure. A 2018 study in Allergy & Rhinology confirmed that the timing doesn’t meaningfully affect how well it works.
That said, a small percentage of people feel mildly drowsy on cetirizine. A meta-analysis of controlled trials found the actual drowsiness rate was only about 1 to 6.5 percentage points higher than placebo, so most people won’t notice anything. If you do feel tired after taking it, switching to a nighttime dose lets drowsiness overlap with sleep instead of your workday. If drowsiness persists regardless of timing, a different antihistamine may be a better fit.
How Quickly It Works
Zyrtec has one of the fastest onset times among over-the-counter antihistamines. In a head-to-head outdoor study published in the Journal of Allergy and Clinical Immunology, cetirizine began relieving seasonal allergy symptoms within the first few hours, outpacing loratadine (Claritin) in both speed and overall effectiveness. Relief holds through the full 24-hour window, which is why once-daily dosing is sufficient.
Taking It Every Day Long Term
For chronic conditions like year-round allergies or hives, many people take Zyrtec daily for months or even years. This is generally considered safe, but there’s one important thing to know about stopping.
In 2023, the FDA issued a warning that some people who stop cetirizine after long-term daily use experience intense, sometimes severe itching. This isn’t the original allergy returning. It’s a rebound reaction that typically starts within a few days of stopping the medication. In reported cases, the median duration of use before this happened was about 33 months, and the risk appeared to increase the longer someone had been taking it. In nearly all cases where people tried stopping and restarting, the itching came back each time they quit. Restarting the medication resolved the itching in about 90% of those affected, and tapering off gradually worked for roughly 38%.
The underlying reason for this rebound itching isn’t fully understood, but the pattern is consistent enough that the FDA now requires it on the label. If you’ve been taking Zyrtec daily for several months and want to stop, tapering your dose gradually rather than quitting abruptly may reduce the chance of this reaction.
People Who Need a Lower Dose
Cetirizine is processed through the kidneys and liver, so people with reduced kidney or liver function may need to take less. The FDA prescribing label recommends a lower starting dose for these groups, since the drug stays in the body longer when these organs aren’t clearing it efficiently. Adults 65 and older fall into this category as well, with a recommended maximum of 5 mg per day. If you have known kidney or liver problems, a healthcare provider can help determine the right dose for you.