If the test is simple, external, as in a breath analyzer then sure. If it involves needles then no. A physician should be able to look at someone and know if they are under the influence of something. Those people need to be fired as an example to everyone else.
Drugs have no place in a work environment.
Pot is legal up here and one day I went in to a Fido phone mart, the employee came out of the back smelling like pot and I left, never to return.