Strain anything hard enough and it will deform/break - at which time you can point at the failure and say "see? There was a defect/issue here!"

Inject enough disruptive chemicals in a finely tuned highly complex chemical process (your brain) and you'll screw it up.

Any existing mental health issues will, of course, become soon apparent. Lacking any (and nobody is mentally perfect), screw with brain chemistry long enough and there will be.