The standard recommendation for Zyrtec (cetirizine) is one 10 mg tablet per day for adults and children 12 and older. That single daily dose is the FDA-approved maximum for over-the-counter use, and for most people dealing with allergies or mild hives, it’s all that’s needed.
Why One Tablet Lasts All Day
Zyrtec works by blocking histamine, the chemical your body releases during an allergic reaction. A single 10 mg dose starts working within 20 minutes for about half of people and within one hour for 95%. That effect persists for at least 24 hours, which is why the label calls for once-daily dosing. Taking a second tablet the same day won’t make it work faster or better for typical allergy symptoms. It will, however, increase drowsiness.
You can take it with or without food, and timing doesn’t matter much. Some people prefer mornings; others take it at night if drowsiness is a concern. The 5 mg dose is also an option if 10 mg causes too much sleepiness or if your symptoms are mild.
Doses for Children
Children need lower amounts based on age:
- Ages 2 to 5: 2.5 mg once daily (half a teaspoon of the liquid)
- Ages 6 to 11: 5 mg once daily (one teaspoon of liquid or one 5 mg chewable tablet)
- Ages 12 and up: 5 to 10 mg once daily, same as the adult dose
Children under 2 should not take Zyrtec without specific guidance from their pediatrician.
When Doctors Prescribe Higher Doses
There is one situation where more than 10 mg per day is used: chronic hives (urticaria) that don’t respond to the standard dose. European allergy guidelines allow doctors to increase a second-generation antihistamine like cetirizine up to four times the standard dose, which would be 40 mg. In practice, the best available evidence only covers doses up to 20 mg daily, and even that evidence comes from small studies totaling just 76 participants. No high-quality data exists for doses above 20 mg.
This kind of dose increase is strictly a medical decision for people with severe, persistent hives that haven’t improved with normal dosing. It is not something to try on your own because your seasonal allergies feel particularly bad one day. If 10 mg isn’t controlling your allergy symptoms, the better move is switching to a different antihistamine or adding a nasal steroid spray rather than doubling up.
What Happens If You Take Too Much
Cetirizine is relatively well-tolerated, even at higher-than-recommended doses. If you accidentally take two tablets in one day, you’re unlikely to experience anything dangerous, but you will probably feel noticeably drowsy. Other symptoms of taking too much include dizziness, headache, and dry mouth. The drowsiness effect scales with the dose, so the more you take, the sleepier you’ll feel.
Serious allergic reactions to cetirizine itself are rare but can include facial swelling, throat tightness, or trouble breathing. These require immediate medical attention regardless of the dose taken. If you or a child takes significantly more than the recommended amount, contact Poison Control at 1-800-222-1222 or use their online tool.
People Who Should Take Less
Your body clears cetirizine primarily through the kidneys. If you have kidney disease or significant liver problems, the drug stays in your system longer, which means even the standard 10 mg dose can build up and cause stronger side effects. A lower dose of 5 mg is typically more appropriate in these cases.
Older adults also tend to clear cetirizine more slowly and are more sensitive to its drowsiness effects. Starting at 5 mg is a reasonable approach. Alcohol and other sedating medications amplify cetirizine’s drowsiness as well, so combining them with even a standard dose can leave you more impaired than you’d expect.
If One Zyrtec Isn’t Enough
Before reaching for a second tablet, consider whether the issue is timing, expectations, or the wrong medication. If Zyrtec seems to wear off before 24 hours, taking it at the same time each day can help maintain steady levels. If it never seems to work well enough, you may respond better to a different antihistamine like loratadine or fexofenadine. Some people also get more relief by combining an antihistamine with a different type of allergy medication, like a nasal corticosteroid spray, rather than increasing the antihistamine dose.
For persistent hives specifically, if 10 mg daily isn’t cutting it after a couple of weeks, that’s worth bringing to a doctor. Dose escalation under medical supervision is a recognized treatment strategy for chronic urticaria, and your doctor can monitor for side effects while finding the right dose for you.