I used k9 advantix II on 2 of my large dogs and also on my great dane. I noticed it took about a week to completely clear them of fleas. I live in Vermont where there is many fleas and tons of ticks. We have a hay field in our back yard and our dogs love to lounge out there where they pick up a lot of ticks. Two of my dogs got Lyme disease before I started using this brand. They had to have shots for the Lyme and my vet suggested we use this to help keep them from getting ticks. They were also bringing ticks into our house. Our dogs didn't have any side affects and it did work but it only worked for about 3 weeks for the fleas and they kept getting ticks in their skin the whole time. The ticks we did find were barely burrowed and they were dead and dehydrated. Apparently that is how it works. The tick dies from ingesting the dogs blood. Our dogs haven't had any Lyme symptoms and I am happy with the results. The only thing I don't like is that it works for 3 weeks rather than 4. They get fleas again before i can apply more. Side Effects No side effects for any of my dogs.