A report by the Natural Resources Defense Council (NRDC), "Poison on Pets II: Toxic Chemicals in Flea and Tick Collars," found that many over-the-counter insect control products for pets, even when used as instructed, can cause "serious health consequences to pets and humans." Many of these products include organophosphate (OP) compounds, which have been used for insect control for decades and are known to have toxic effects. Most immediate health problems come from not using these products properly, but there is some evidence that more insidious health problems may arise from chronic exposure. http://bit.ly/2AxAE84