Zyrtec is a second-generation antihistamine used to combat allergy symptoms such as sneezing, rashes, runny noses and other respiratory symptoms. Antihistamines are notorious for causing drowsiness. However, second-generation antihistamines--Zyrtec and Claritin, for example--are less likely to have this side effect. The key to determining the time of day in which to take Zyrtec depends on the way the medication affects you.

Monitor Yourself

In the first days of taking Zyrtec, monitor how you feel. Because of the possibility of drowsiness, you should begin taking the drug at night, unless your doctor says otherwise. Keeping a log or journal may be best. Record trouble getting to sleep, restlessness and insomnia. If you experience any of these symptoms, stop taking the Zyrtec at night. Instead, take that nightly dose the very next morning. Look for signs of drowsiness, dizziness and fatigue. These are signs that you should take the drug in the morning.

Call Your Doctor

When experiencing symptoms that prevent functioning awake in the day and sleeping at night, call your doctor. The side effects of Zyrtec are not meant to be "suffered through." Your doctor can work with you to tweak the dosage or find another medication that has fewer side effects. Fortunately, Zyrtec symptoms are mild and do not affect all users. Once you figure out the best time to take the medication, stick to that schedule. Skipping doses throws off the scheduling and may cause you to feel the side effects at inappropriate times.

Don't Stop Monitoring

You may not keep a log, but stay alert to changes while you are on Zyrtec. The sudden onset of symptoms after being on the regimen for a while warrants a trip to the doctor. It could be a sign of a larger problem. Consult your pharmacist when adding any new drug to you daily routine, as there may be drug interaction issues to address. Also, stop the drug immediately once severe side effects appear. These include severe skin rashes, trouble breathing, seizures and vertigo or if the whites of the eyes turn yellow.

Source:

FDA information on Zyrtec

More Information on Zyrtec