Well, showing indications of harmful effects is not that hard...
<cancer
memory loss (somewhat controversial - conflicting studies exist
Personal experience: I almost killed a carload of kids - including myself - in the 70s while under the influence. For me it enhanced an existing flaw of mine where I can become absorbed in one thing to the exclusion of everything else (I'm firmly in the Pres. Ford catagory). On this occasion, with a minor distraction, I forgot I was driving a car (on Highway 1, south of Oxnard, CA - windy road, sea cliffs).
