Johnson's 4fleas Spot-on for Medium Dogs 10-25kg is a fast-acting spot-on that kills fleas on contact, as well as flea larvae in the surrounding area. It is for the prevention and treatment of flea infestations and for the treatment of biting lice. This veterinary-approved treatment provides up to 8 weeks of protection, with each of the 2 vials offering up to 4 weeks of protection. Fleas are killed within one day of treatment.