Research continues to prove what many have been saying for years: cannabis can stay in the system for a long time after the substance was used, and therefore cannabis blood tests are not a fair way to tell if someone has just used cannabis.

A new study titled “Residual blood THC levels in frequent cannabis users after over four hours of abstinence,” which appeared in the Drug and Alcohol Dependence journal, provides more evidence that alcohol can stay in the bloodstream long after a high has passed or after cannabis has been used. The study was conducted by researchers affiliated with the University of British Columbia, and then their results were published in an official study.

“Some stakeholders worry that current per se limits may criminalize unimpaired drivers simply because they use cannabis,” the researchers explained. “We conducted a systematic review of published literature to investigate residual blood THC concentrations in frequent cannabis users after a period of abstinence.”

So far, the study shows that if more than 2ng/ml are detected in the blood from cannabis use, they can persist for an extended period of time, so it’s not fair to look at that amount in the bloodstream and claim someone was recently exposed to cannabis, and it is certainly not fair to determine the person is still intoxicated. 

Authors reported: “[I]n all studies where participants were observed for over a day, blood THC [levels] in some participants remained detectable during several days of abstinence,” with some subjects continuing to test positive for up to 30 days. Some subjects also demonstrated a so-called ‘double hump’ pattern “where their THC levels rose toward the end of the week after an initial decline.

“The studies in our review consistently demonstrate that positive blood THC levels, even levels over 2ng/ml, do not necessarily indicate recent cannabis use in frequent cannabis users.”

Why The Research Matters

This research could be huge for safety laws and driving violations, as zero-tolerance traffic policies usually determine that if a driver has even trace levels of THC, they are impaired and should not be driving, and can therefore be arrested and charged with intoxicated driving. Since THC can be in the system even days after smoking, it is not fair to treat this infraction the same way that alcohol is treated. There needs to be a more definitive test to determine whether people are driving high.

“Many jurisdictions have per se limits for THC, often 2 or 5 ng/mL, that make it illegal to drive with THC above the ‘legal limit,’” the study claims. “People who use cannabis regularly develop partial tolerance to some of its impairing effects. Regular cannabis users may also have persistent elevation of THC even after a period of abstinence.”

Until more testing can be done on a fair method of screening drivers, there won’t be a definitive answer as to who is driving high and who simply used cannabis days ago. Hopefully, more research will come when more studies are allowed on a federal level.