What an accidental double dose may mean, when not to increase the dose on your own, and how to respond safely in the United States. The practical answer For most adults, the OTC label for plain Zyrtec is one 10 mg tablet in 24 hours. Taking two tablets is outside the label. A one-time accidental total of 20 mg may mainly cause added sleepiness in an otherwise healthy adult, but the safest next step is…