When it comes to mind altering drugs (and how the brain functions in general) there is a plethora of things that we don't know. For example, the way that anesthesia actually functions is largely a mystery. We just know that it works most of the time.
At the moment we can say that on occasion taking certain antidepressants can potentially cause suicidal thoughts to either manifest themselves or become worse. You also have to consider what kind of antidepressant is being used (TCAs, NDRIs, NRIs, SNRIs, SSRIs (probably the ones people think of the most including Prozac, Zoloft, and Celexa), NaSSAs, etc). All of these different classes of antidepressants function in different ways and within each class each drug has its own consequences, some good and some not so good.
The medical community is always evolving and discovering that some drugs we have been prescribing for years are harmful (think Vioxx). Some function well as something completely different (Rogaine started off as Loniten, a drug meant to treat hypertension). And some should actually be taken by a totally different subset of people (Thalidomide used to be taken for morning sickness until it was discovered that the r enantiomer treats morning sickness and the s enantiomer causes birth defects, and you really can't separate it. It is now taken by men with prostate cancer and a couple of other diseases, but usually not women of childbearing age).
Unlike the medical community that can and should change its mind every once in a while, our constitution and laws we make have to be able to be applicable no matter how many times the medical community comes out with new knowledge that may or may not still be true in 20 years.
Then there's the issue of correlation not equalling causation.
Looking at the above graph we can clearly see that the average global temperature is a function of the number of pirates. As we can see, we can't simply assume that psychos are like that because they take meds. Maybe, but we don't know for sure, and a lack of evidence denying it and the fact that many psychos have this in common doesn't make it any more true than the number of pirates affecting average global temperature.
Our laws have supersede the emotion of the day. Yes, when people are needlessly killed we will be upset. But, the thing about a republic is we need to be able to say "yes, that was terrible, but this event can not cause our laws to infringe on our rights".
We can say the the freedom of speech has caused people to be killed. It certainly caused many in the civil rights movement to be murdered. Having everybody agree and not have contradicting opinions would certainly cause a few less people to be murdered. But there are some things more important than my life, and not having anything infringe on any of my civil liberties is one of them (and not just the second A, I'm a stickler for all of them).