To the O/P: The proper vitamin to use for urine color is B2 (Riboflavin), not B12.
This worries me. My aunt (a registered nurse with 30+ years of experience and no reason to lie) and uncle (an MD with more experience than my aunt) AND a lab tech have all three told me that Riboflavin can cause a false positive for THC, even in patients who would otherwise be clean.

Curiously, I had a lab tech who recently attended a training conference tell me that Nexium (the little purple indigestion pill) could also cause a false positive, to the point where if someone tests positive for THC after disclosing a drug history including nexium, many labs will report back inconclusive. He said when taking a history, if the patient discloses nexium, they almost expect a positive result for THC.