Mother Nature has already put a psychotropic drug in the drinking water, and that drug is lithium. Although this fact has been largely ignored for over half a century, it appears to have important medical implications.
See NYTimes article.
This is a fascinating article by Anna Fels, a psychiatrist who teaches at Weil Cornell Medical College in NYC. Lithium's atomic weight is close to 7. It was in the original 7-Up and some have suggersted that "7-Up" stands for the salutary effects of Li on mood. Go figure!