(NewsTribune) Alcohol exits the body in a matter of hours. Cocaine can take a couple of days.
Cannabis has no exact timetable.
The cannabis compound detectable in urine samples is evidence of past use, not actual impairment, said Dr. Alan Shackelford, a Harvard Medical School-trained physician who founded Amarimed of Colorado, a clinic in Denver focusing on medical marijuana. He has used cannabis to treat several children with rare seizure disorders who don’t respond to other medication.
The body metabolizes marijuana differently than most other substances, Shackelford said. Those who smoke or ingest marijuana feel a high for a short time from a psychoactive chemical in cannabis called THC, but its metabolites can remain in urine samples for weeks.
There are many THC and other cannabis-related chemicals, but the two Shackelford notes are delta 9 THC, which causes the high, and carboxy THC, a metabolite that does not impair the user and remains in the body long after the high is gone.
“Here’s the thing that’s problematic. Blood or urine tests don’t look at delta 9 THC,” he said.
Drug testing companies, including Quest Diagnostics, use urine tests that seek evidence of past marijuana use — carboxy THC.
Five years ago, Shackelford sent a letter to the working group in Colorado that was considering rules that would allow police to judge driver impairment by using marijuana metabolite levels in blood or urine.
“You can’t do that. Cannabis is different (than alcohol),” he said in early September.
Because carboxy THC is fat soluble, it remains in the body longer than other substances, such as alcohol or opioid painkillers, which are water soluble. The time it lingers varies by individual, Shackelford said.
Factors that affect detectable levels include how often a person uses marijuana, how much body fat the person has, whether the person smokes or digests a cannabis product, and the concentration of the product itself, he said.
If someone who does not normally partake in pot chooses to smoke or eat a cannabis product on a Saturday and takes a urine test on a Monday, they might not test positive for carboxy THC, Shackelford said.
“If they do show positive … they are probably not impaired and are perfectly capable of doing any task,” Shackelford said.
But medical marijuana patients often use cannabis daily, Shackelford said. Daily users can test positive for weeks after their last dose. Medical patients have asked Shackelford what to do if they have to pass a drug test.
Shackelford tells them that products claiming to remove THC from the body don’t work. A few patients have reported back that stopping for 10 days helped them pass a drug test. Shackelford cautions it’s not a scientific study — few studies have been conducted in the United States because of federal restrictions.