Drug tests.....neccesity or invasion of privacy?

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I'd like to know what people think on this topic. Several good workers have lost their jobs to random drug tests. If a person is reliable, shows up everyday, does their job and does it well, should they be fired for what they do on their own time? Pot stays in your system sometimes for months. Should a person lose their job because of a joint they smoked at a party a week ago? I think it is an invasion of privacy. If there is cause to drug test, due to injury or negligence..that is reasonable. But not random...too many good people lose their jobs for no good reason.