if you are prohibited from using drugs as a condition of probation, then you are probably not going to be given a pass on failing the drug screen; you would need to hire an attorney as soon as possible.....
You are not allowed to fail a drug test while on probation. However, exactly what happens after you fail your test is an open question. While the best thing to do is pass all drug tests by not taking illegal substances, your attorney may be able to work out a deal that everyone can live with in the event you happen to fail a test. Whatever you do, don't stop reporting to your PO and keep your attorney informed so they can help you no matter what happens.
When you meet with your probation officer, he or she will most likely give you a drug test. Most terms of probation include the statement "Avoid injurious and vicious habits, especially alcoholic intoxication and narcotics and other dangerous drugs unless prescribed lawfully." This means Do not drink or take any drugs, ever, for any reason. No party is worth spending months in jail after you've been caught drinking or drugging.
I've been blogging about probation issues on my website lately. Please read http://www.clarktowne.com/DUI-Blog/ for more information.
Best of luck.