Some people have made the argument that knocking first gives criminals time to dispose of evidence. This of course only applies if the evidence happens to fit through the s-trap of the suspect's toilet. In that case, here is a way to make your neighborhood drug free. Start knocking on every door. If you hear a flush, you have either gotten someone off drugs or you can be confident that he is eating his fiber.
You might be able to flush a few joints down the can. Bong pipes and meth labs will not go down the drain however. A dealer with a big stash had better hope that he can think of a better way to hide or ditch the goods than using a plumbing fixture. And the fact of the matter is that junkies and dealers hate having to flush the stash. Even if the police don't make the arrest they still have caused someone to flush a lot of expensive dope.