The best active ingredient to kill fleas is by far Frontline Plus. It contains fipronil, which kills adult fleas and stops eggs from hatching, and also methoprene, which kills larvae. It's important to use Frontline Plus every month as directed to keep your pet protected against fleas.

There are a number of active ingredients that can kill fleas, but the most effective one is probably pyrethrin. Pyrethrin is a naturally occurring chemical found in the chrysanthemum flower, and it's very effective at killing fleas, ticks, and other parasites. It's also relatively safe for use on pets and humans. Other common active ingredients for killing fleas include permethrin, imidacloprid, and fipronil. These chemicals are all insecticides, meaning they kill insects by disrupting their nervous system. They're all relatively safe to use, but it's important to follow the directions carefully and to avoid contact with skin or eyes.