After being placed under arrest for DUI, the police will typically read you your implied consent rights.  Typically the arresting officer is determining whether to request a sample of your blood, breath, or urine.  In cases where the officer believes you may be under the influence of an illegal or controlled drug the officer will ask for urine.  The urine will be collected and sealed.  The urine will then be screened for drugs.  The results of this urine test can and will be used against you in court.