Adult Side Effects

• Adults taking Zyrtec may experience drowsiness, fatigue and a dry mouth. If you experience a dry mouth, try placing an ice chip in your mouth or chew some gum.
  
  

Children Side Effects

• Children taking Zyrtec may experience more severe side effects, including abdominal pain, diarrhea, headaches, sleepiness and a sore throat. It can also lead to coughing, wheezing and nosebleeds.
  
  

Pre-existing Conditions

• Tell your doctor your complete medical history before taking Zyrtec. If you have kidney or liver disease, you will likely need a reduced dose of Zyrtec.
  
  The effects of Zyrtec on an unborn fetus are unknown. Tell your doctor immediately if you become pregnant or if you plan to become pregnant. Zyrtec does enter into breast milk, so do not take it if you are breastfeeding.
  
  

Interactions

• Some foods or other drugs may interact with Zyrtec. You should not drink alcohol or take sleeping pills and other sedatives, tranquilizers, muscle relaxants, antidepressants, narcotic pain medication or high doses of theophylline. Tell your doctor about any medications or over-the-counter supplements you are taking.
  
  

Overdose

• Always take your medicine as prescribed. If you experience severe side effects, talk to your doctor about reducing your dose or switching to another medication. If you suspect you have overdosed on Zyrtec, call your doctor immediately. The primary symptom of a Zyrtec overdose is extreme sleepiness. Children may become irritable and restless before becoming sleepy.