Comfortis (spinosad) tablets are a prescription medicine for cats used to kill fleas and prevent flea infestations. They are chewable, beef-flavored tablets that are given once a month. The active ingredient, spinosad, works by attacking the nervous system of fleas, causing them to die quickly. Comfortis is effective for one month and starts killing fleas within 30 minutes of administration. Recommended for cats 14 weeks of age and older, weighing 4.1 pounds or more.