What can motive false positives and negative on a drug tryout?

i was recently given a random drug examination at my pain management doctor. they prescribe to me, talwin nx, flexerill, vicodine, but i didn't show positive for opiates. i was positive for marijuana and amphetamines, neither of which i do. i do thieve a number of other OTC and prescription meds.
Answers:
Check this info (most popular)
http://www.drugtestfriend.com/falseposit…
...also for each of them you can check at NIDA (The National Institute on Drug Abuse)
http://www.nida.nih.gov

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