Generally speaking, it is not a good idea to use a cat flea collar on a small dog. There are a few reasons for this. First, the dosage of the active ingredient in the collar is calibrated for a cat, not a dog. This could potentially lead to your dog becoming sick if they were to wear the collar for an extended period of time. Additionally, most flea collars are made with materials that may be harmful to dogs if ingested, such as plastic or rubber. If your dog were to chew on the collar, they could ingest these materials and become sick as a result. Finally, many dogs are allergic to the chemicals used in flea collars, which could lead to irritation or even anaphylactic shock. For all of these reasons, it is generally best to avoid using a cat flea collar on your small dog.