If you're referring to K9 Advantix, I have been using iton my 3 yr. old Springer. The Vet suggested reapplying every 3 weeks during the summer months. Just last year he was covered with fleas and we didn;t know why. It was the first and only time he ever had them. The Vet suggested he may have come in contact with a "storm" of fleas. Anyway, we used the Advantix and the next day (literally 24 hrs) he had no fleas on him at all. It really is great stuff. We also notified the company because he should have been protected and they sent another 6 month supply to the house - no charge.