Most industries have their poison be it stimulants, pain killers, alcohol, tobacco so it is not just the oilfield. In our industry I would assume 15-25% are functioning alcoholics (believe the US rate is 10%), something like 80% of Silicon Valley CEOs are on the newest stimulant whatever it is at the time.
I have no idea about drug test since I have had random ones required since 18, was a good job and did not want to ruin it. Now my job and one of my customers make me take random ones. Oddly enough every time I travel to Colorado for work I happen to get chosen for the "random" drug test.