Harmony Pet Home Diffuser Refills for Dogs are unplugged. Contains 2 refill cartridge. Each diffuser refill continually releases a safe pheromone for dogs for up to 30 days, providing 1 month of calming benefits when they need it the most. The P.E.T. pheromone mimics the maternal pheromone from momma dog to soothe her pups. Dogs of all ages recognize this calming pheromone for life.