If it's legal, there is no need to test for it and/or it can be ignored if it shows up in a test. The issue would be if they are stoned or drunk at work, not if it shows up in their system from use last night or last month.
Not so. If your work has a drug policy, at least in Oregon, it doesn't matter if you have a prescription or not.
Likewise, although you can get a prescription for it and get a grower's license to grow it, it doesn't matter. If you get busted, you are still busted.
"On October 19, 2009, the United States Department of Justice (USDOJ) issued a memorandum to provide clarification and guidance to federal prosecutors in states that have authorized the medical use of marijuana. The guidance makes it clear that USDOJ is committed to prosecuting enforcement of the Controlled Substances Act, but that, as a general matter, federal resources in states with medical marijuana laws should not be focused on individuals who are "in clear and unambiguous compliance with existing state laws providing for the medical use of marijuana." Prosecution of commercial enterprises that unlawfully market and sell marijuana for profit will continue to be a priority of USDOJ
"The guidance from USDOJ does not make the use of medical marijuana legal under federal law, and does not create a defense to a federal prosecution for a drug related offense."
Oregon Medical Marijuana Act - Legal Information