The standard maximum dose of Zyrtec (cetirizine) for adults is 10 mg in 24 hours, which is one standard tablet taken once daily. Most people should not take more than this in a single day. The 10 mg dose is what the FDA approves for over-the-counter use, and doubling up because your allergies feel worse is not recommended without medical guidance.
Standard Adult Dose
Zyrtec is designed as a once-a-day medication. Adults and children 12 and older can take either 5 mg or 10 mg per day depending on symptom severity, though most people use the full 10 mg tablet. You take it once, with or without food, and it covers you for the full 24 hours. There’s no benefit to splitting the dose into two 5 mg tablets taken hours apart; the drug stays active in your body long enough that a single dose is sufficient.
If 10 mg isn’t controlling your symptoms, taking a second tablet on your own is not the answer. The extra dose primarily increases drowsiness and side effects without a proportional boost in allergy relief at the OTC level.
Doses for Children
Children’s limits are lower and depend on age:
- Ages 2 to 5: 2.5 mg once daily, which can be increased to a maximum of 5 mg per day (either as a single dose or split into 2.5 mg every 12 hours). Do not exceed 5 mg in 24 hours.
- Ages 6 and older: 5 mg to 10 mg once daily, with a maximum of 10 mg in 24 hours, the same ceiling as adults.
Children under 2 should not take Zyrtec without specific instructions from a pediatrician.
Lower Limits for Older Adults and Kidney Problems
If you’re 65 or older, the recommended maximum drops to 5 mg per day. This isn’t just a suggestion. Your body clears cetirizine more slowly with age, so a standard 10 mg dose can build up and cause more pronounced drowsiness and dizziness.
The same 5 mg ceiling applies if you have moderate kidney or liver disease, because your body relies on the kidneys to eliminate the drug. People with severe kidney impairment should not take cetirizine at all. If you have any kidney issues and aren’t sure where you fall, this is worth a direct conversation with your doctor or pharmacist before taking even a standard dose.
When Doctors Prescribe Higher Doses
There is one scenario where people legitimately take more than 10 mg daily: chronic hives (urticaria) that don’t respond to standard doses. European allergy guidelines suggest that doctors can increase a nonsedating antihistamine up to four times the standard dose, which would mean up to 40 mg of cetirizine per day. In practice, clinical studies have only tested up to 20 mg daily, and even that evidence comes from small trials totaling just 76 patients.
This is strictly a doctor-supervised situation. If your hives aren’t responding to one Zyrtec a day, the path forward is a medical appointment, not self-prescribing extra tablets. Higher doses significantly increase drowsiness and other side effects, and there’s no high-quality data on safety beyond 20 mg.
What Happens If You Take Too Much
Cetirizine is relatively well-tolerated even in doses above the recommended amount, but “well-tolerated” doesn’t mean consequence-free. The primary symptom of taking too much is pronounced drowsiness. People who’ve accidentally taken extra doses report feeling noticeably sleepy within a short time. At higher overdose levels, symptoms can include dizziness, rapid heart rate, headache, and agitation or confusion.
In one case documented by Poison Control, a toddler who ingested 60 mg (six times the adult dose) became very drowsy during dinner. While cetirizine overdoses are rarely life-threatening, large overdoses can potentially affect heart rhythm. If you or a child has taken significantly more than the recommended amount, contact Poison Control at 1-800-222-1222.
Why One Tablet Lasts All Day
Part of the reason people wonder about taking more is that their symptoms seem to return before the next dose. Zyrtec is formulated as a single daily dose because cetirizine remains active in the body for a full 24-hour cycle. If you feel like it’s wearing off in the afternoon, the issue is more likely that your allergen exposure has increased (you went outside, pollen counts rose) rather than the medication leaving your system. In that case, reducing your exposure, using nasal spray, or switching to a different antihistamine may work better than doubling up.
Alcohol and Drowsiness
Cetirizine causes less drowsiness than older antihistamines like diphenhydramine (Benadryl), but it still causes some, particularly at higher doses. Alcohol amplifies this effect. If you’ve taken even the standard 10 mg dose, drinking can make you significantly more impaired than either one would alone. This matters especially for driving or operating equipment.
Stopping After Long-Term Use
If you’ve been taking Zyrtec daily for months or years, be aware of a withdrawal effect the FDA flagged in a safety communication. Some people experience intense, widespread itching within a few days of stopping the medication. The reported cases were rare but sometimes severe enough to need medical treatment. This itching is not your original allergy symptoms returning; it’s a rebound reaction. If you’ve been on Zyrtec long-term and want to stop, tapering gradually rather than quitting abruptly may help, though you should discuss the approach with your doctor.