For me, my comeback is the one that shuts them up the most: "At least I don't sound like some drugged up loser." And by drugged up, I mean someone who has been drugged against their will. Cause the people who say this are "Friends"-watching white lib mental hospital believers. Whenever someone comes up with that, I'll win the argument, simply saying, "At least I don't sound drugged up." Cause that insult shuts em up.
Now I don't know what lead them to say that or who they are but my guess is that they chucked as they walked away at how lame that comeback was. In fact, if you believe that was a good one, then you are loony and don't know what your talking about. It ranks right up there with "I know you are but what am I?"
From what you stated, it sounds like your "friends" are people who have personally experience with mental hospitals as you claim they have been "drugged against their will." If that is the case, then they have valid point of view to base their belief of mental hospitals. Question is, what experience do you have to base your opinion?
Seeing that I don't know what the conversation was about that lead them to their statement and little information about you and them, I can only make rough assumptions.
Hypothetically, if they are a person that you have actually been friends with and they are/were put in a mental hospital but you have not, a better comeback would be, "Let me know when your medication wears off." You see, this would also imply that they are the real loon and that they may not be thinking clearly. Granted, it's not my best work but just about anything is better than your comeback.