I would say, if you're not actually doing the drugs on company property, or acting like you're on drugs while on property, you're mostly* fine.
Someone clearly toked up in the car on the way to work the other night (one of the overnight shift folks) and the TMSC just reeked of skunky-ass weed. Everyone pretended not to notice, as far as I can tell.
*Just don't get hurt. I think they drug test you if you get hurt on the job. Then you can likely kiss any workman's comp and your job goodbye.