Your best bet....
Go to your employer, and say:
"Look, you obviously think that I am good enough for the job, because I'm here. Now you're saying that you don't trust me..ie, you're making me take a drug-test. How do you think that makes me feel? It makes me feel that YOU don't trust me...and that can cause paranoia...I'll be thinking more about your lack of trust, than I will about the job at hand. So, why don't we just forget all this, so called 'drug test' malarky, and carry on as is. If, in the future, you feel that I am not performing to the required level, then sack me for that...but, in the mean time, as long as I'm not breaking any laws within your premises and in YOUR time - LEAVE ME THE FUCK ALONE!!!"

That might work..
It's what I would say....sort of...

If I was an employer, I wouldn't drug test anyone - I would judge them upon their performance within my employ...if they fail to meet the standard, then I'd have a chat with them, then go from there. Similarly, as long as no other employee was in danger, I'd rather have a happy workforce, than one that was continually wondering if I was gonna spring a drug-test on 'em...

..but that's me, eh?