Jennifer, If the clog is "down stream" of a branch to another drain, you'll just blow all the gunk back up that other drain.
Also if you have old drain pipes, you stand a very good chance of breaking something. I even managed that with just a plunger over used. You can also blow stuff up the roof vent pipe and give the birds a big thrill! Even plungers aren't meant to provide lots of downward pressure, but to just "jiggle" things about to loosen the clog. Unlike supply pipes, drains are not meant to take pressure. Tom Fowle
