The ingredients in dog collars are highly toxic to dogs when ingested and even when they lick the flea collar residue found on the dogs fur.
The dog can get what's known as propoxur poisoning which is deadly if not treated soon by a vet.
When your dog licks on or chews the flea collar the dog can get amitraz toxicity so never allow your dog to chew on or eat a flea collar and if they do you need to take the dog to see the vet.