Is it okay to take 20 mg zyrtec/ daily?

Probably: Although the usual dose of zyrtec (cetirizine) for an adult is 10 mg daily, under the direction of a physician, some patients may need more. It would be important to discuss the dosing with your physician as another approach to treatment may be indicated. Avoid drinking alcohol and be aware that drowsiness may be more likely at higher doses.

Yes: Most people can tolerate doubling the dose. In fact , we use even higher doses for those with hives.