I did the NIH IRTA starting in 2011 at NIEHS, and was never drug tested. I think it would have come across as faintly ridiculous in any case since I was doing bioinformatics. Maybe it's different if you have patient contact.
(Edit) I have never used illegal drugs, but I'd be more troubled by an alcoholic or tobacco-addicted doctor than one using marijuana according to its typical patterns of use. What is the point of these "well don't smoke weed then" replies?