My border collie pup is now a year and 4 months old. He came from a bad situation and was crawling with worms when we got him. For the past year I have been worming him an average of every 6 weeks alternating between drontal and panacur. When I do the drontal worming there is always at least a few large roundworms produced in his poo. I've never seen worms produced with panacur interestingly, so perhaps panacur kills worms differently, or maybe it isn't even working? Anyway is this persistent worm problem very abnormal or has anyone ever had this with their dog?