Re: While doing a urine test on an employee, can anything be taken to give untrue results?

I looked this up in a Substance Abuse textbook and also asked one of our 
substance abuse experts in our department.  It seems there are no substances 
that you can take to avoid detection of a drug.  However, what can be done is 
basically to drink lots of water to try to dilute out the substance of abuse 
such that it is no longer detectable.  The most common test in emergency rooms 
for example, is the use of the traditional TLC  (thin layer chromatography) 
"toxicology screen" to detect what are presumably high doses of drugs. A TLC tox 
screen is NOT sufficiently sensitive for diagnosis of low-dose use of illicit 
drugs. When TLC is used, false negatives are common. TLC sensitivity is in the 
range of 2 micrograms per mL. Thus if you dilute out the drug by drinking lots 
of water or using diuretics you may avoid detection. In forensic settings TLC is 
not admissible and one must use more sensitive methods such as Gas 
chromatography or Mass Spec. On a final note, there are substances which one can 
ingest which can come out as illicit in a tox screen.  For example, ingestion of 
a large quantity of poppy seeds (as in poppy seed muffins) will register as an 
opiate positive tox screen (since poppy seeds have morphine).

References
Substance Abuse: A comprehensive textbook
second edition
Williams and Wilkins

hope this helps,
cheers,
gabriel vargas, md/phd