I personally have taken so called mind altering drugs in the past.. Since then I have been totally clean for a period of a couple a years.
Since then I had suffered a bout, lasting about a year, a period of total depersonalisation and a feeling of not even being the person I am, feeling as if I was living life behind a camera, life was happening around me yet I wasn't the one living it. Also at the same time I was suffering debilitating anxiety for no particular reason at all. There was no pre-trauma that I can associate with it, nothing, 1 day it started and I don't know why.
I can tell you straight up, mental illness is a far, far, far more debilitating disorder then any drugs I have ever taken.. Driving a car on pot was nothing compared to driving with an anxiety disorder. The effects of a hallucinagenic were nothing compared to being overcome with fear to point that even leaving your own house was the scariest thing imaginable...
I would be far more worried, having been through both, of a pilot with a mental disorder who cannot function at all, compared to someone whose abilities *MAY* be diminished, yet may not through smoking a bit of pot or indulging in a bit of ecstasy on the weekends..
In other words, there is no point telling the medical officer your history, you'll only ruin a chance at a successful career. The laws are bias against drugs which the UN (USA) consider illegal, alcoholism and mental illness(as long as youre on another mind altering drug, anti depressants and the like) is fine.