Most unnecessary deaths of dogs that occur due to flea collar poisoning are the result of an allergic reaction to the active ingredient. Even if your dog is perfectly well at the time the device placed around his neck, observe your dog closely for the first few days. 

Most packages containing flea collars or other similar-type pest-fighting devices usually carry a printed caution on the front: “Do not allow children to play with this collar”.

When I was younger and living in San Diego, California, a mixed breed lab died a painful death which was eventually traced to the flea collar it was wearing.