Our family dog (she was 8 years old when she got what turned out to be her final dose) had seizures after she had frontline applied to her. My mom gave it to her, and she said right after she put it on, the dog shivered when it was applied. A few hours later she started having seizures for the first time. She went to the vet and they put her on medication to slow them down, but it didn't help her much. We started giving her some flax oil pills which helped for awhile but my mom wouldn't change her diet to a raw food diet (which may have helped) and she started to get worse after a few months. We finally had to put her to sleep a month before her 9th birthday because the seizures had made her blind and she kept having more every day. I would be wary of using it on older dogs to be sure. Of course the vet said there's 'no way' that could have caused them, but how do they know? They don't!