Tags:  Tetrahydrocannabinol; per se limits; systemic review; blood THC; cannabis; abstinence

 

Thumbnail: Blood THC does not accurately predict recent cannabis use.

 

The problem: Driving under the influence of cannabis can give rise to psychomotor impairment and an increased likelihood of a crash. To minimize these risks, many states have imposed blood THC limits, often 2 or 5 ng/mL, on drivers. However, because THC accumulates in body fat before being slowly released into the bloodstream, regular cannabis users may have high blood THC even after abstaining from cannabis. Blood THC is hardly indicative of impairment, yet it leads to the criminalization of unimpaired users behind the wheel.

 

The study: This systemic review included 6 studies related to “cannabis,” “blood,” “concentration,” and “abstinence” that measured the blood THC of 142 frequent cannabis users in total after 4 hours of abstinence. The objective was to describe for how long frequent cannabis users have any detectible blood THC, blood THC over 2 ng/mL, and blood THC over 5 ng/mL after usage.

 

Main results:

• Although blood THC declined over time, it remained detectable over several days of abstinence.

  • In one study, half of the 10 participants that were monitored for 30 days of abstinence had detectible blood THC up until the 30th day.

  • In another study, while occasional cannabis users had detectible blood THC for only about 4 hours, frequent users had detectible blood THC for over 30 hours.

• Some frequent cannabis users had blood THC over 2 ng/mL after 6 days of abstinence.

  • In one study, 2 of 25 frequent cannabis users that were monitored for 7 days of abstinence had blood THC >2 ng/ml on the 7th day.

• Some frequent cannabis users had blood THC over 5 ng/mL after 1 day of abstinence.

  • In one study, 2 of 12 frequent cannabis users had blood THC >5ng/mL 30 hours after smoking a single cannabis cigarette. Meanwhile, all the occasional smokers had blood THC <5 ng/mL after 2 hours of smoking the cigarette.

 

Conclusions:  Blood THC is not a reliable indicator of recent cannabis use. In other words, frequent cannabis users with blood THC levels >2 ng/mL and >5 ng/mL did not necessarily use cannabis recently.

 

Why this is a good study:

• This review makes for a current and comprehensive analysis of residual blood THC levels in frequent cannabis users after abstinence.

 

Why this isn’t a perfect study:

• There is a lack of standardization between the studies in review. For example, while some studies measured for blood THC, others measured for plasma THC. One study was conducted in a treatment center rather than in a securely monitored facility. There is also reason to suspect that one of the participants in this study used cannabis during the abstinence period. Its results must be taken with a grain of salt. More generally, the review was limited by a lack of available research. The overall participant pool is not necessarily generalizable to the entire population.

 

What this study adds:

• Blood THC may not be the best tool for detecting recent use of cannabis.

 

What it doesn’t:

• More research is needed to determine the relationship between residual blood THC and psychomotor performance. More research is also needed to identify factors that give rise to prolonged blood THC.

 

Funder:  None.

 

Author conflicts:  None.