Zyrtec (cetirizine) is taken once a day. The standard adult dose is 10 mg every 24 hours, and you should not take a second dose sooner than that. It’s designed as a once-daily antihistamine, and one dose provides a full day of allergy relief.
Standard Dosing for Adults and Children
For adults and children 12 and older, the recommended dose is 5 to 10 mg once daily. Most people take the 10 mg tablet. You can take it with or without food, and at whatever time of day works best for you.
For younger children, the doses are lower and vary by age:
- Ages 6 to 11: 5 or 10 mg once daily, depending on symptom severity.
- Ages 2 to 5: 2.5 mg once daily, which can be increased to a maximum of 5 mg per day. That 5 mg can be given as a single dose or split into two doses 12 hours apart.
- Ages 6 months to under 2 years: 2.5 mg once daily. For children 12 to 23 months, the maximum is 5 mg per day, split into two doses 12 hours apart.
Why You Shouldn’t Double Up
Taking more than one dose in a 24-hour period increases the risk of drowsiness, which is the most common side effect of cetirizine at higher doses. In one case reported to Poison Control, a 75-year-old woman accidentally took an extra two tablets (30 mg total instead of 10 mg) and became noticeably drowsy. A 12-month-old who accidentally swallowed about 60 mg of liquid cetirizine also became sleepy enough that his mother noticed him dozing off at dinner.
These examples are reassuring in one sense: cetirizine overdoses tend to cause sedation rather than something more dangerous. But heavy drowsiness can be a real safety issue if you’re driving or operating machinery, and taking more doesn’t improve allergy relief in a meaningful way for most people.
What If One Dose Isn’t Enough?
If your standard 10 mg dose isn’t controlling your symptoms, the answer usually isn’t to take it more often. For seasonal or year-round allergies, adding a nasal corticosteroid spray or switching to a different antihistamine is a more effective next step than doubling your cetirizine.
There is one exception. For chronic hives (chronic urticaria) that don’t respond to the standard dose, international allergy guidelines allow doctors to increase the dose of a nonsedating antihistamine up to four times the standard amount. For cetirizine, that means up to 20 mg daily has been studied in small trials, and it did improve itching and welts in people with severe, treatment-resistant hives. This is considered off-label use and should only be done under medical supervision. No high-quality data exists for cetirizine doses above 20 mg daily.
Best Time of Day to Take It
Zyrtec works for a full 24 hours regardless of when you take it, so the time of day is flexible. A clinical trial comparing morning versus evening symptom relief with cetirizine found equivalent coverage across both time periods. That said, the study also noted that taking it in the morning slightly increased reports of daytime sleepiness. If drowsiness bothers you, taking your dose at bedtime is a simple fix: you sleep through the peak sedation and still get full coverage during the day.
If You Miss a Dose
If you forget to take Zyrtec at your usual time, take it as soon as you remember. If it’s already close to the time for your next dose, skip the missed one and go back to your regular schedule. Don’t take two doses at once to make up for it. Since cetirizine lasts a full 24 hours, being a few hours late won’t leave you completely unprotected.
People Who May Need a Lower Dose
Older adults and people with kidney or liver problems clear cetirizine from their bodies more slowly, which means the drug stays active longer and side effects like drowsiness are more likely. Starting at 5 mg instead of 10 mg is a common adjustment for these groups. If you have significant kidney disease, a lower dose or less frequent dosing may be appropriate, something worth discussing with a pharmacist or doctor.