I see nothing wrong with it. Plus, even if you do injure yourself on the job and you are stoned when it happens, it doesn't much matter. I have injured myself at work completely sober and was unable to report it. All injury reports filed result in a drug test, so they would have refused to pay the medical bill anyway and I would end up losing my job...

Sometimes I feel like I should go to work stoned sometimes, I'm fucked no matter what if I get hurt. I feel I'm a little to accident prone to be stoned there though...