Dogs given 1-2 applications of topical insecticides per year were 1.6 times more likely to have bladder cancer compared with dogs not receiving any applications. Dogs given > 2 applications of topical insecticides per year were 3.5 times more likely to have bladder cancer compared with dogs who did not receiving any applications of insecticide. 
There was a significant dose response relationship between number of applications per year and risk of bladder cancer. This risk was further enhanced in overweight or obese dogs, suggesting that carcinogens in commercial flea and tick dip products are concentrated and stored in body fat, then excreted in the urine over long periods of time.