How do I use boric acid to kill fleas? Boric acid is generally sold at hardware stores, usually it will be labeled as a flea killer. It will come in like a shaker kind of container, much like this or really any other flea powder. You also achieve the same thing that boric acid does by mixing borax, four parts of borax to one part salt, using it in the same way you would use boric acid. And boric acid essentially your just going to take in this little shaker, and this is used on carpeting, and you'll take and you'll just sprinkle it over carpeting o.k. And you'll kind of mash that in. You want it to get as deep in to the carpet as possible, o.k. And then you're going to let it sit for a day. And what that does is that'll dessicate any adult fleas and eggs and stopping them from growing or killing adults, stopping them from laying eggs any further. After that day is up, you're going to have to vacuum the boric acid back up. Now the boric acid is relatively harmless. No one I've ever heard of ever having any problems with it, hurting their animals or anything, but as always be careful when using anything like this. You never know what the effects it might have. And there you have it, as simple as that. We have used boric acid to treat our carpet for fleas.