Flea collars cause seizures, diarrhea and vomiting in some dogs as they respond to the toxins on the surface of the collars, states the Washington Post. While not every dog has this reaction, it is a common phenomenon when the dog is allergic to the flea collar.
Both flea collars and topical chemicals can cause severe reactions in pets, notes PETA. Some researchers link their use to cancer, although more research is necessary to establish a connection with certainty. In some cases, the side effects from the products are worse than having fleas in the first place, notes the St. Louis Post-Dispatch.
Some natural alternatives have the chance to make flea collars unnecessary. Washing the pet's bed in hot, soapy water weekly, vacuuming the house weekly, throwing the contents of the bag into the outside trash, bathing the pet regularly with natural cleansers and using a fine-toothed comb to find and eliminate fleas are all remedies that don't introduce any toxins into the pet's system. It is important to look at the active ingredients on the packaging of any chemical that one plans to use on a pet, states Good Housekeeping.