Blindman is close about the false positives. Drug tests are usually done in 2 stages, first immunoassay then any non negative sample goes to GC/MS. There are no false positives in GC/MS and an extremely low number of false positives in immunoassay but once it goes to GC/MS they will know 100%.

Prescriptions for opiates will not help you get out of a positive for THC.