I had used this on both my dogs. About the 4th time, my pug/westie mix started jerking towards her hind/tail area (I had put most on the back of the neck and a small portion at the base of her tail).Since we had used this product before, we first thought she had hip or back problems so took her to the vet and specialist. After $1500 and 4 trips to the vet, including major pain medication she was better after a few days so assumed it was just a joint/back/hip problem that got better. The next month, right after using Advantix again, the same thing happened which led to the obvious conclusion it was an alergic reaction. I wished I had known it could be washed off with dish soap (per the earlier review) because she had to spend another 2 days until she stopped reacting to the treatment.