It really depends on the attitude of your judge, the prosecutor, your probation officer, and the programs available in your jurisdiction.More lenient jurisdictions will sometimes use a two-strikes rule.  Namely, the first time you test dirty, they just tell you not to do it again.Rehab oriented jurisdictions will look for programs to send you to in an effort to help you over come any addiction issues.Hard-core jurisdictions will simply revoke your probation and send you to jail.  If they don't impose the original sentence ordered, they will sometimes just order a short jail trip as a sanction for the positive urine test.I know these are three general answers, but these are the ranges of what could happen.  To know the attitude of your jurisdiction, see if you can arrange for a free consultation with a couple of criminal defense attorneys in the jurisdiction where you case is pending.  They can help you narrow down which of these might happen based on their experience with your local court participants.