Drugs usually stay in the system for 2-3 days. I.e it can not be traced in urine after that time. However, it remains in Hair follicles for as long as 6 months. I believe if you smoked 6 months ago you don't need to worry as the drug effect would be gone by now. However, if you shave your head, they might revert to testing the hair on your body. So its useless to shave off your head.
