My dad used to take non-prescription meth during his student days for better focus (this was way before Adderall). He did not have any addiction problem with it

My dad used to take non-prescription meth during his student days for better focus (this was way before Adderall). He did not have any addiction problem with it and used it only as a performance enhancer, like before exams. He could and did discontinue at will. Our family life was and is as normal and balanced as it gets. He turned out to be a decorated scientist. He is in his 80s now and has no known health issues related to meth. Just an anecdote, but there it is. I suspect much of the harm caused by meth is not by the meth itself but the impurities and contaminants that result from synthesizing them at under-equipped (both in terms of know-how and physical equipment) home labs.