If you use mouthwash like you're supposed to - rinse your mouth and spit it out - then it's not going to cause you to fail a drug test and (unless you have something in your mouth, like dentures or bridgework, that can trap some mouthwash) it should be clear for purposes of even a breath test after no more than fifteen minutes or so.
If you're an alcoholic, go to the drug store and get a mouthwash that doesn't use alcohol as a base, then throw away your old stuff.

