Although it is most often used to kill roaches, borax can kill fleas effectively. Borax comes in different forms, but powdered borax is easy to spread over carpet and in tight spaces where fleas sometimes hide.
Borax kills bugs by dehydrating them. It can take weeks to completely kill an adult population, but borax can also kill eggs, making it a powerful option for eradicating pests. Homeowners can kill fleas by first cleaning the home as much as possible to get rid of spots where fleas can hide. After spreading the borax powder across the room and working it deep into the carpet, homeowners should let it sit for 24 to 36 hours before vacuuming it away.