Zantac is an H2-Blocker and Zyrtec is an antihistamine. They do not have overlapping dosage strengths, but they are both available in the syrup dosage form, Zantac as 150 mg/10 mL and Zyrtec as 5 mg/5 mL. Glaxo Wellcome manufactures Zantac and Pfizer manufactures Zyrtec and the container labels do not appear similar.