Mandatory Drug Screen urine test

office visit was second visit to this Physician for Medication refill- ((office visit required for refill (done by nurse practitioner) charged for mandatory drug screen G0431 $650 (said it was to make sure prescribed meds were in patients system). What is the law (Las Vegas, Nevada) for mandatory drug screen and who can say it is mandated for refills of prescribed meds and how often is it billable?
Thanks