The main active ingredients in Zyrtec-D are cetirizine and pseudoephedrine. Cetirizine is an antihistamine, while pseudoephedrine is a decongestant. These ingredients are chosen to relieve swelling and allergic reactions in the nose, and thereby reduce stuffiness and similar symptoms such as sneezing or a runny nose.
Antihistamines work to reduce the effects of the body's immune system in terms of the production of histamine, which is created in response to substances the body identifies as foreign and potentially dangerous. Excess histamine leads to symptoms such as rashes, swelling, itchy membranes, sneezing and the excess production of fluids, such as tears and mucus. Therefore, reducing the amount of histamine in the system helps to combat these symptoms.
Decongestants such as pseudoephedrine work by narrowing the blood vessels in the nose to decrease stuffiness, swelling and congestion. This enables the user to breathe more easily, which can help to relieve other allergy symptoms such as watering eyes. It is the inclusion of a decongestant component that makes the Zyrtec in question carry a "-D" suffix.
While Zyrtec is generally noted as being effective in relieving the symptoms of seasonal allergies, it has a comprehensive list of side effects that should be considered before use. As with many antihistamines, these can include drowsiness, so users should be careful not to drive or operate heavy machinery after taking this medication until they know if it will affect them.