Usage of Zyrtec may lead to several side effects such as impairing of your reactions or thinking. Be aware in a situation, where you need to be very alert and awake such as driving of vehicles. Drinking alcohol could greatly multiply several unexpected serious effects of taking Zyrtec. Do not confine your doctor, if you are a regular user of medication, which makes you feel sleepy (like other allergy medicine or cold, sleeping pills, narcotic pain killers, medicine for seizures, muscle relaxers, anxiety or depression). They can compound to sleepiness induced by Zyrtec. Consult your general practitioner, in case the symptoms never show any sign of improvement and gets worse. In case, if you have been caught up with fever, then it is for sure you must have to consult your doctor