Advantage II Topical Flea Treatment for Extra Large Dogs (over 55 lbs) kills all flea life stages and prevents fleas on a treated dog from infesting your home. Starts killing fleas within 12 hours of initial application (kills re-infesting fleas within 2 hours) and keeps working for up to 30 days. This spot treatment is waterproof and works through contact, so fleas don’t have to bite to die.