Scientific studies have shown that Zyrtec is moderately effective and well tolerated in dogs with allergic skin conditions [4, 7]. However, do not give your dog Zyrtec-D, a decongestant containing pseudoephedrine pseudoephedrine HCl Pseudoephedrine is used for the temporary relief of stuffy nose and sinus pain/pressure caused by infection or other breathing illnesses. as one of the active ingredients since it is toxic to dogs [9].