Residual blood THC levels in frequent cannabis users after over four hours of abstinence A systematic review..
Tetrahydrocannabinol (THC), the primary psychoactive component of cannabis, causes psychomotor impairment and puts drivers at increased risk of motor vehicle collisions. Many jurisdictions have per se limits for THC, often 2 or 5 ngmL, that make it illegal to drive with THC above the legal limit. 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. Some stakeholders worry that current per se limits may criminalize unimpaired drivers simply because they use cannabis. We conducted a systematic review of published literature to investigate residual blood THC concentrations in frequent cannabis users after a period of abstinence. We identified relevant articles by combining terms for cannabis and blood and concentration and abstinence and searching MEDLINE, EMBASE, PsycINFO, and Web of Science. We included studies that reported THC levels in frequent cannabis users after more than 4 h of abstinence. Our search identified 1612 articles of which 8 met our inclusion criteria. After accounting for duplicate publications, we had identified 6 independent studies. These studies show that blood THC over 2 ngmL does do not necessarily indicate recent cannabis use in frequent cannabis users. Five studies reported blood THC >2 ngmL (or plasma THC >3 ngmL) in some participants after six days of abstinence and two reported participants with blood THC >5 ngmL (or plasma THC > 7.5 ngmL) after a day of abstinence. Blood THC >2 ngmL, and possibly even THC >5 ngmL, does not necessarily represent recent use of cannabis in frequent cannabis users.