Never experienced it but this is what I found...
The primary active compound is catnip, nepetalactone, is chemically similar to hallucinogens.
Catnip is not toxic to pets. http://www.citycats.biz/catnip.htm
I really can't tell you one way or the other however I can share with you the experiences I have had with dogs and catnip filled toys.
My dogs, I have 5, will destroy an cat nip filled mouse in short order. Even my toy poodle will rag and gum one to death. The G.S will eat them cloth and all.
I don't give them to the dogs they just find them laying around where the cats have left them and gooble gooble..!!!
None have even sustained any ill effects although the cloth covering of the catnip mouse can't do their digestive tracts any good.
I just have never seen an ill effect of the cat nip on my dogs but that doesn't mean it couldn't upset a tummy or two.
Maybe someone on the board will have better information for you.
WB
(THIS IS TAKEN FROM A DIFFERENT DOG FORUM SO THE LINK IS NOT PROVIDED)