It will be very interesting to see what effects the new legislation will have.
The legislation makes it an absolute offence to have drugs or alcohol in your body and sets specific limits for alcohol.
If you are tested positive, then you are guilty of a criminal offence and there is no defence (which is why a zero-tolerance level of 0.2% has been set). It seems unlikely that anyone found guilty would be returned to work......... and any airline that did not report "unfit" flight crew could be considered guilty of conspiracy - which reduces the scope for in-house rehabilitation.
What I find interesting in this thread is the lack of mention of "drugs". These are not effectively defined in the legislation and no "cutoff" levels are set. The term embraces a wide variety of illicit drugs as well as legally obtainable drugs that may impair your "function" - for example, anti-depressants, beta-blockers, heart drugs etc. Drugs are in widespread use (overtaking alcohol in traffic offences) and it seems niaive to ignore their use.
The body metabolises alcohol quickly (and even manufactures it, which is why the .02% cutoff level is set). However, the body metabolises other drugs much less efficiently and if "zero tolerance" levels are set (as with alcohol) as seems likely, this could mean that an individual may test positive several days after imbibing.
The police will test if they are reliably informed (by workmates etc) that an offence may have been committed.
The industry really does need to take this issue seriously and not pretend that cabin and flightdeck staff don't take "drugs". Pro-active random testing seems unavoidable.