Claritin and Zyrtec are two antihistamines that should not be taken together, states BJC HealthCare. Patients can take either Claritin or Zyrtec, or take each one on alternating days, always drinking lots of water. Patients who have prostate problems or difficulty holding their urine should not take either medication.
Patients who need to remain alert while taking Claritin should be careful as they may experience sleepiness, which is a common side effect. Dry mouth is another side effect common to the long-acting antihistamine, according to BJC HealthCare. Before taking Claritin or Zyrtec, patients should give their health-care professionals a complete list of their prescription and nonprescription medicines, including vitamins and herbs.