Like others have said, you can go manic while on meds, but that shows they aren't working.

For me, I was on antipsychotics for my schizophrenia, and the doctors assumed they would work as a mood stabilizer, too. Actually, the antipsychotics ended up MAKING me depressed, but they didn't realize that. They then put me on antidepressants to fix my depression, and those made my mood wildly unstable, to the point that I started rapid cycling and had a very bad mixed episode (agitated depression with obsessive thoughts).

They realized that for whatever reason, my body doesn't process antipsychotics as mood stabilizers, so they put me on lithium instead. It's been working pretty well so far. :P

EDIT: I should also say that I have bipolar 2, so I've only ever been hypomanic. I've heard from some people with bipolar 1 that they stop having manic episodes and major depressive episodes, but they still sometimes have hypomania or depressive moods.