Maybe he means ED?
Most anti depressants cause ED because they lower the sex drive, and from prolonged use(or in your penises case, lack of use), shrink the penis. The lower serotonin will mean your generally more depressed, however it will not make you have ED, and I doubt it would lower your sex drive by much. If you take medicine for your serotonin (and you won't die if you don't take it) try going off of it for a week and try masturbating again. You should feel more sensitive to everything as well as a whole lot more horny and turned on. Worth noting is that when you cum, you get a small boost of serotonin in your brain for a little while - just thought it's interesting - The cost of being more horny is that you feel worse so it's your call. I'm not sure if there are ways to get antidepressants/serotonin without the side effect of lowering your sex drive but if you find anything, tell me!!!