Quote Originally Posted by Burnt Toast
Whether or not the zinc was responsible for the negative test results is a big question mark. Due to the fact that another factor was involved: dilution.
I rechecked the abstract from the Oxford website. The word "diluted" is not mentioned. I am too cheap to buy the full paper, as I would probably not understand most of it anyway. I do not know why I added the word, "diluted". See what abstaining from weed does to me?

Urine samples from acute marijuana smokers were also obtained in order to study the effects of zinc supplements on THC drug testing. All urine drug testing was performed using ELISA detection kits manufactured by Immunalysis. Both zinc sulfate and zinc supplements are effective in interfering with the detection of all three drugs by Immunalysis drug detection kits. Also, no suitable method could be established to detect zinc in urine samples. Zinc can be an effective adulterant in urine for some illicit drugs that are commonly screened under routine drug testing.
Zinc Reduces the Detection of Cocaine, Methamphetamine, and THC by ELISA Urine Testing

I would guess that it will only be a matter of time for zinc to be added to list of detectable adulterants. Spot testing was developed in 2013. Old news, but hey, I'm old too.