If the drop coincides with May 22, you were probably hit by the Google roll out of "Penguin 2.0" Supposed to be much tougher on spammy links.
I am in a similar situation although for some keywords, I have just dropped a few slots. I am pretty new to this so I am sure you will get an answer from one of the very helpful experts here, but you might want to take a look at you link profile using OSE. It seems you have quite a few "cialis" links and that can't be good. Also see if you have gotten any warnings in GWMT. If you did, then it is what they call a "manual" action meaning a human at Google assessed a penalty. There is a process for reconsideration if this is the case. It could however just be the algorithm change.
Either way you should seriously look at getting rid of those links. I will be interested in the community responses here.
Best of luck, I know it is a bad feeling!