I suggest you read up well on the product: blog.foxassurances.com/post/2012/07/20/Communiqu%C3%A9-ANSES%3A-Mise-en-garde-collier-antiparasitaire-SERESTO
Read the testimonials also: blog.foxassurances.com/post/2012/04/09/On-teste-pour-vous%3A-Seresto-collier-anti-puce-longue-dur%C3%A9e
So monitor your cat the first 2 weeks to see if it tolerates the product.