The company I work has random drug testing, which occurs about once every three months. A couple of weeks ago I was called in to be tested. Fine with me, as I have nothing to hide and I feel that the company has every right to call me in for three reasons. One, it's a known fact that I'm on antidepressants. Two, I have had knee surgery three times, and who's to say I'm not hooked on pain killers(which I'm not.) And lastly, I have admitted to have used recreational drugs when I was in my twenties.
The reason we have this mandatory testing is that we work on jobs for the government. Is it fair? I think so as what they are looking for are illegal drugs and others that aren't prescribed to the person using them. It just strongly advocates personal responsibility.
Where I work we make maintenance parts for the railroad. It's not unreasonable to expect employees to work unimpaired. After all, we have people's lives in our hands.