The first question is: "What is the nature of the job? Would using drugs affect his job performance and possibly risk the lives of others?" (i.e. healthcare workers, pilots, ship captains, bus drivers, etc.)

If his job is not a job that could risk the lives of others, then the second question is: "Who am I to judge? Is it any of my business what another person does in his own private time?"

I think the best policy in dealing with people in the workplace is to confront the person (in private) you have a problem with or let go of the issue. If working with the person has become an issue, then ask your boss to be reassigned, or you can choose to look for another job.