Capstar Is A Pill You Give Your Dog Or Cat To Kill Adult Fleas On Your Pet. Pet must be 2 lbs. of body weight or greater and 4 weeks of age or older. Not for human use. Keep out of reach of children. Capstar only kills the adult fleas on your pet. You will need to treat your house, yard and car to prevent reinfestation. You may use Capstar with spot on flea products. You may use Capstar as often as once per day. Capstar begins working within 30 minutes and kills 90% of adult fleas within 6 hours. Blue pill contains 11.4 mg of nitenpyram. For Dogs or Cats 2 to 25 lbs. One tablet.