Diphenhydramine or Benadryl, can cause a false positive for PCP. As far as causing a false positive for opiates, I do not think so. I took 3 benadryl last night and took a 6-panel urine test this...
One of the panels tests specifically tor opiates. If you have a script from your doctor you will present it to the MRO. If you do not have a script, and have taken someone Else's pills for whatever...