The argument is that tampering with your brain chemistry is more likely to set off people in our general population who aren't wired all that well in the first place. In other words, it brings out psychoses that might not otherwise manifest.
That it worsens psychoses that do.
“Tampering” in just what manner? Alcohol is known to fuel more rages than pot is. Yet used in a responsible context, alcohol is a wonderful comestible.
Again, the name of the game is responsibility. People are not meant to live either in vacuums or in rubber rooms.