This might be the greatest intro ever to a major media story on marijuana. From USA Today:

On a typical weekday, stockbroker Irvin Rosenfeld has a marijuana cigarette before work, then goes to his firm's smoking area for another after he gets to the office. By day's end, he usually has smoked more than a half-dozen joints — and handled millions of dollars' in clients' holdings.
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His firm, Newbridge Securities, supports his use of marijuana and says it hasn't hurt his performance.

In so many ways, the mere existence of Irv Rosenfeld demonstrates the fundamental wrongness of typical anti-marijuana rhetoric. That he receives his medicine directly from the federal government proves that they've always known the truth, despite subsequently pretending not to. His success demonstrates that marijuana, even in large doses, can be part of a healthy and productive lifestyle.

Of course, Irv is one of a shrinking handful of federally approved medical marijuana patients. The security he enjoys is highly anomalous:

None of the states with medical marijuana laws requires employers to make accommodations for the use of the drug in the workplace, says Bruce Mirken, spokesman for the Marijuana Policy Project.

Yet, there are legal gray areas for companies, say employment lawyers such as Richard Meneghello of Portland, Ore., who does seminars for companies on the topic.
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Meanwhile, there are questions about whether medical marijuana laws would offer any protection to employers if a worker who used marijuana to treat pain ended up injuring others or making a mistake on the job. It's unclear whether such an incident has occurred.

Medical marijuana has been legal in California for 10 years and it's "unclear" whether an incident has occurred. I think that says it all. Come what may, the truth will always be that competency is best determined by conventional means and not through urinalysis.

Unfortunately, the problem goes beyond that of competent employees establishing the trust of sympathetic employers:

Scott Seidman, a Portland lawyer who represented Columbia [a company sued for firing an MMJ patient], says the company had to maintain its drug-free workplace policy because it is a federal contractor.

Once again, the federal prohibition against medical marijuana is central to the problem. Unless, of course, you're one of the few people who receive medical marijuana in the mail each month from the same government that says there's no such thing.