Employment Screening

I recently took a pre-employment screening that I was upfront about that would test positive for THC. That was not a concern for the employer. About an hour ago I received a follow up questionnaire from the lab testing my sample that asked if I had a medical card and/or a prescription for Marinol. Is this standard practice for a positive THC test or should I be worried that there is something showing in my system that I don't know about?