Can you give an 8 yr old cough medicine with Zyrtec?

Dogs can have either Zyrtec or Robitussin DM (or CF). These medications are for two different purposes, they can't be used interchangeably. Zyrtec is an allergy medication, and Robitussin is a cough suppressant.

Your vet can advise you of the proper medication and dose for your dog, based on it's weight and the condition your dog has.