Stress Center Peer Support

Is being told "you're so paranoid" by a coworker all the time a form of harassment?

"harassment: the act of systematic and/or continued unwanted and annoying actions of one party or a group, including threats and demands"

It's pretty much up to the individual. If you feel that your coworkers behavior is harassment you should speak to him/her or your boss. Even if it's not technically harassment, your coworker should not be making a comment like that.

Depends on the circumstances. If you are minding your own business, then it is inappropriate in my view. But if you are constantly expressing worry etc. there may be a reason for the comments.