I support random drug and alcohol testing. I think about 1% of pilots checking in for work should be tested. That's a high enough level to deter coming to work impaired, which is the objective.
There is no room for impaired pilots in the cockpit - period.
My biggest concerns are false readings and third party mischief such as giving a pilot a marijuana brownie, which has been done by a vindictive ex, who then reported the pilot.