Coming up dirty on your first test is sort of expected. There should be no punishment, but it will be used as a baseline and there is no excuse for it happening a second time. You say the charges are false (of course!) but admit to having an addiction. So, you need to consider it this way: you have possessed drugs before and not been caught. This time you were charged but actually didn't have drugs. In the end, you were bound to get caught sooner or later and the root issue is your addiction. While it may be none of my business, I suggest you seek treatment (AA and NA are free) for your own long term best interest. If you get thrown off PTI, things will get worse for you quickly. Do the right thing for the short term and the long term and find a better way to live. Good luck.
I think it also depends on what substance you tested positive for and when you first drug test was. If you test positive for a short lived substance like cocaine and your first pre-trial drug test was several days after your arrest, you may find yourself in trouble. Now, drugs like opiates and marijuana can stay in your system for a longer time and so you may "fail" your first urine screening.
If your pre-trial release conditions are violated, you can be re-arrested and even denied bond completely because of the nature of the underlying charges - trafficking!