The Feliway Multi-Cat Unscented Calming Diffuser Refill is vet-recommended, drug-free and clinically proven to reduce tension and fighting among cats in the home. By mimicking the natural feline appeasing messages (pheromones), Feliway Multi-Cat sends 'harmony messages' to help your cats get along more effectively and avoid tension, conflict and fighting in the home; most cat owners see a 70% reduction in conflict in 3 weeks. The multi-cat refill includes 1 refill vial (diffuser head not included). Each refill vial contains enough Feliway Multi-Cat pheromones to last you about 4 weeks. For maximum effectiveness, you should replace the calming diffuser refill every 4 weeks and your diffuser every 6 months.