Admitting to recreational drug use
will disqualify you from holding a medical at the first instance - about 10 years ago one of my students admitted to smoking a bit of pot on the weekends
Mind you, admitting to illegal activity during an interview with a government medical officer could be viewed as failing the underlying IQ test anyway
I am assuming you have never been caught in posession and that there is no official record of your past silliness.
I won't encourage you to lie in your aircrew medical but I think the question you will be asked is "Have you, since your last medical or in the last 12 months, used any illicit drug or narcotic?"
See if you can go 12 months without - perhaps then
1/. you know you have the self-discipline required;
2/. you will have the clear head required for the very intensive (and expensive) training and
3/. you can answer the question truthfully.
Best of luck.