The active ingredient in Zyrtec is cetirizine and is a safe drug to use in dogs for allergies. Typical dosage is 0.5 mg/pound of body weight, with a maximum dosage of 20 mg. Cetirizine can cause side effects and does interact with some other medications and should only be administered under the supervision of a veterinarian. Check with your vet before giving cetirizine to your dog, and never give a dog Zyrtec D because it contains pseudoephedrine.