Should the use of medical marijuana be allowed in the workplace?

WalMart worker of five years was terminated for the use of medical marijuana on the job. worker twisted his knee, while working at WalMart and was given a mandatory drug/alcohol screening. the positive results for marijuana, resulted in a termination from WalMart. worker has an inoperable brain tumor and his doctor recommended the marijuana to help ease his pain. WalMart did not see it this way and he was fired. WalMart stated his use of marijuana on the job, places other WalMart workers and customers at a safety risk. the worker has sued WalMart. Question: should the use of prescribed medical marijuana be allowed in the workplace or is it a threat to the safety of the other workers?

Source: Reuters