Hi Stephanie,   Great question! No, side effects definitely don't have to mean you are allergic to a drug. There are many side effects that are much milder than an allergic reaction. In most cases, side effects lessen or go away over time. However, it is also possible to be allergic to a drug.   The type of symptoms you are having will suggest whether you are having routine side effects or a sensitivity to the drug (allergy). Common allergy symptoms include itching of the mouth, throat, wheezing, swelling in the throat, etc., while the side effects of Zyrtec D are more likely those such as:   - Sleepiness (most common side effect) - Dry mouth - Sore throat - Dizziness - Headache - Nausea   Whatever symptoms you are having should be reported to your doctor, who can advise you on a course of action.   To your health, Kathi