“EPA is intensifying its evaluation of all spot-on products…adverse reactions reported range from mild effects such as skin irritation to more serious effects such as seizures and, in some cases, death

-Environmental Protection Agency

This stuff is passed out like candy by many many veterinarians. However,

“before use of these products on weak, aged, medicated, sick, pregnant or nursing pets, or on pets that have previously shown signs of sensitivity to pesticide products, EPA recommends that a veterinarian be consulted.”  – Full Article

Dogs who are fed kibble diets are often, if not sick, weak. These diets are, again, sold to consumers by their veterinarians. Brand names like Purina, Eukanuba, Science Diet etc are high in carbs, preservatives and “animal byproduct” the lowest quality meat possible, often road kill, sick animal carcasses and who knows what else. These dogs are also more likely to be over vaccinated and over medicated with other pesticides such as heartworm medications.  As such these dogs immune systems are compromised and they are at higher risk of experiencing side effects from the spot on flea treatments. These side effects may be the “minor” skin burns or irritation,  but they might also be cancer 5 years down the line.

also see: this article and this study