Seresto® is a convenient, easy-to-use, non-greasy, odorless collar that kills and repels fleas and ticks for 8 continuous months. Seresto® is veterinarian-recommended and available without a prescription. Item Dimensions. Item Dimensions Length 2 X 15 IN Click to see full answer. In respect to this, how long do Seresto cat collars last? eight months Also, is Seresto safe for my cat? The Seresto cat collar has some demerits, it will not be suitable for every cat. Moreover, the cats can show an allergic reaction after you put this collar. The reason behind it includes either cat is allergic to flumethrin or imidacloprid or the material of collar does not suit the cat. Considering this, do Seresto cat collars work? If your furry friend is struggling with painful ticks and fleas, then the Seresto Cat collars are here to give them an easy and fast relief. Though this collar works best for the cats and most vets recommend t for the cats, but in some special cases, it can be used on your dog too.How tight should Seresto collar be?According to the product manual, the Seresto collar should be tightened firmly enough so that the loose end of the collar pulls through the clasp at the end, and you should be able to fit two fingers between the collar and neck.