Flea Collars for dogs and cats are usually pretty safe however if the dog or cat has more sensitive skin then they might experience side effects such as scratching, redness, and possible hair loss.
But most times dogs and cats will never experience those side effects but they could be allergic to the flea collar or just have more sensitive skin.
If your dog or cat or other animal does experience those side effects then you should remove the flea collar and just continue to bathe them in flea shampoo or dip them in a flea dip.
Flea Collars stay effective for up to 90 days and after 90 days the flea collar becomes less effective and needs replaced.
Also flea collars do not really kill off existing fleas but instead the flea collar is to prevent more fleas from getting onto your pet.
So if your pet already has a flea infestation they need a flea bath or flea dip to kill off the existing flea infestation.