Effexor is just a standard anti-depressant (similar to Cymbalta or Wellbutrin). Plenty of functioning, well-adjusted people take these drugs (and see psychiatrists) and are still productive members of society.
Of course, I can’t speak to your particular situation– I don’t know how long you were seeing this girl, how this information “came out”, or what exactly “erratic and flaky behavior” means (doesn’t sound like it has anything to do with power dynamics, but if this thread called it to mind, then maybe). It’s certainly possible that she did/does not have her depression under control. But there are many who do, and it’s unfair to judge all people based on the medications they take. Keep in mind that those who take medication are 1000 steps ahead of those who should, and don’t.