I worm my dogs every month , the life cycle of many worms vary greatly the round worm life cycle is 28-35 days round worms in severe cases in children can cause retinal problems which I worry about , hookworm life cycle is from 15-18 days and tape worm 14-21 days , Im not sure that 3 monthly worming is sufficient these days , but the drontal packet says 3 monthly is fine , so many conflicting opinions .