Did your vet specifically say that cetirizine itself was ok as long as it was not combined with another drug for sinuses?

I'm not familiar with this drug but will say that as long as your vet said that ingredient was ok, as long as the product you're giving lists ONLY that as the active ingredient, you should be fine. If it was for sinuses, it would be combined with another drug, which should be listed on the label.

For example... Loratadine is commonly known as Claritin and is safe to give to dogs. Claritin comes in two forms: Claritin, and Claritin D. Claritin D includes pseudoephedrine in addition to the loratadine, which is contraindicated for pets (I normally tell owners "The D stands for DON'T!"). So, Claritin by itself is ok to give, but Claritin D is NOT.

I hope this makes sense.... but, when in doubt, CALL YOUR VET.

EDIT: So, in your case, the purple box only indicates cetirizine HCl as the active ingredient and no others. So as long as your vet said cetirizine was safe to give, I would say you're safe to give that product to your pet.

EDIT2: I realize I have the tendency to overdo things but I just like making sure you understand why vs just giving you a cut-and-dry answer