The PetLock® 4M - Flea & Tick Prevention for Dogs is a safe and effective way to rid your dog of biting insects in the house or yard. It works fast to kill fleas and ticks on your pet and premises. Then, it continues to work to prevent pesky pests on your pet, home, and lawn. PetLock® has a solution for all pest problems! Large dog is suitable for dogs 21 to 55 pounds.