Can I use Allegra-D in the place of Zyrtec-D?

The primary ingredient in Allegra and Zyrtec are different, but are used for the same thing. The "D" is the same decongestant ingredient, I think allegra has 240mg, zyrtec has 120mg.

You may be better off asking the pharmacist if he has any generics for Allegra D or separate cetirizine and pseudoephedrine tablets you can take together.

Because, pseudoephedrine can be used to illegally manufacture methamphetamines (also known as meth), in the US federal regulations require that it be sold behind the pharmacy or service counter. It is available without prescription, but ID and quantities are recorded to assist law enforcement.

Why don;t you ask the doctor who wrote the prescription? Surely they would know better than we would.

Check with your doctor's office.

ask your doctor. would you really risk your health by following advice found here?

wikipedia:

Cetirizine, prominently marketed under the brand name Zyrtec among others, is a potent second-generation antihistamine used in the treatment of hay fever, allergies, angioedema, and urticaria. It acts as a selective antagonist of the histamine H1 receptor.

Fexofenadine, sold under the trade name Allegra among others is an antihistamine pharmaceutical drug used in the treatment of allergy symptoms, such as hay fever and urticaria. Therapeutically, fexofenadine is a selective peripheral H1-blocker.