The standard dose of Zyrtec for adults is one 10 mg tablet per day, and the label says not to take more than one tablet in 24 hours. That single daily dose starts working within about an hour and provides relief for a full 24-hour period, which is why taking more is rarely necessary for typical allergy symptoms.
Standard Doses by Age
Zyrtec (cetirizine) is a once-daily antihistamine. The recommended doses break down by age:
- Adults and children 12+: One 10 mg tablet once daily. Maximum: 10 mg in 24 hours.
- Children 6 to 11: 5 to 10 mg once daily. Maximum: 10 mg in 24 hours.
- Children 2 to 5: 2.5 mg once daily, which can be increased to 5 mg per day. Maximum: 5 mg in 24 hours.
These limits apply to the over-the-counter product. If your allergies aren’t controlled at the standard dose, taking a second tablet on your own isn’t the recommended next step.
Why One Tablet Lasts All Day
Cetirizine has a half-life of about 8.3 hours, meaning it takes that long for your body to clear half the drug. But its antihistamine activity outlasts its blood levels. A single 10 mg dose starts blocking histamine within 20 minutes in about half of people, and within one hour in 95%. That effect persists for at least 24 hours, which is why Zyrtec is labeled as a once-daily medication. Doubling up doesn’t meaningfully extend the duration of relief; it mainly increases the chance of side effects.
What Happens if You Take More Than One
The primary side effect of taking extra Zyrtec is drowsiness. In one case reported to Poison Control, a 75-year-old woman accidentally took two extra tablets (30 mg total) and became noticeably drowsy. In another, a toddler who ingested roughly 60 mg became sleepy within an hour. Cetirizine is considered relatively safe compared to older antihistamines, but higher doses reliably increase sedation, and that sedation can impair driving and coordination.
If you accidentally took a second tablet, you’ll likely feel sleepy but probably won’t experience anything dangerous. If a child has gotten into a bottle, or if someone has taken a large number of tablets, contact Poison Control (1-800-222-1222) for guidance.
When Doctors Prescribe Higher Doses
There is one scenario where higher doses are used: chronic hives (urticaria) that don’t respond to standard treatment. European allergy guidelines suggest that doctors can increase a second-generation antihistamine up to four times the standard dose for stubborn hives. For cetirizine, that would mean up to 40 mg per day. Small clinical trials have found that 20 mg daily can improve hives and itching in people whose symptoms didn’t budge at 10 mg.
This is off-label use, meaning it goes beyond what the manufacturer’s label recommends. No high-quality studies have tested cetirizine at doses above 20 mg. If your hives aren’t responding to one tablet a day, a doctor can decide whether a higher dose makes sense for your situation and monitor you for excessive drowsiness.
Alcohol and Other Sedating Substances
Even at the standard 10 mg dose, cetirizine can cause mild drowsiness in some people. Adding alcohol or other sedating medications amplifies that effect. If you’re already feeling drowsy from one tablet, combining it with a glass of wine or a sleep aid could make the sedation significant enough to affect your balance and reaction time. This matters more for older adults, who tend to clear the drug more slowly.
If One Tablet Isn’t Enough
When a single daily Zyrtec isn’t controlling your symptoms, taking extra tablets isn’t the most effective solution. You have better options. Adding a nasal corticosteroid spray targets nasal congestion and inflammation in a way that antihistamines can’t. For itchy or watery eyes, antihistamine eye drops work faster at the site than an oral tablet. Switching to a different antihistamine sometimes helps too, since people respond differently to each one. If you’ve layered these approaches and still feel miserable, that’s worth a conversation with an allergist who can evaluate whether something else is driving your symptoms.