There are a few occasions when you will be medscreened. Before appointment to a safety critical post. Before promotion to another safety critical post. For cause after a safety of the line incident (including suicides). On suspicion of being unfit through drink or drugs. On a random basis.
Another tool is the random breath test for alcohol which is used quite frequently when booking on for duty.
The random tests are in my opinion a good thing. Initially my reaction was it is a bleedin cheek etc etc. However, any test that protects my arse when accusations are flying about is fine by me.