Does your employer have an Employee Assistance Program (EAP). If so, it's usually connected to your health insurance and you can contact them directly without having to talk to your immediate supervisor/HR. The EAP is obligated to confidentiality by law. If your employer is part of the Federal Drug-Free Workplace Act, then they MUST have an EAP.
I'm absolutely no expert nor do I claim to know anything for sure, but it's definitely worth looking into. I wish I utilized my employer's EAP (had I only known how it worked!). I probably would not have lost my job if I did.
Look around here:
http://www.dol.gov/asp/programs/drug...tance/menu.htm