PMID: 32841811 DOI: 10.1016/j.drugalcdep.2020.108177
This originally appeared at https://pubmed.ncbi.nlm.nih.gov/32841811/
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 ng/mL, 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 ng/mL does do not necessarily indicate recent cannabis use in frequent cannabis users. Five studies reported blood THC >2 ng/mL (or plasma THC >3 ng/mL) in some participants after six days of abstinence and two reported participants with blood THC >5 ng/mL (or plasma THC > 7.5 ng/mL) after a day of abstinence.
Blood THC >2 ng/mL, and possibly even THC >5 ng/mL, does not necessarily represent recent use of cannabis in frequent cannabis users.
Cannabis is the second most widely used recreational drug worldwide after alcohol.(Peacock et al., 2018) Acute cannabis use is associated with cognitive deficits and psychomotor impairment.(Crean et al., 2011; Desrosiers et al., 2015; Broyd et al., 2016) There is also evidence that cannabis increases the risk of motor vehicle crashes (MVCs).(Asbridge et al., 2012; Rogeberg and Elvik, 2016) Despite these risks, many North Americans drive after using cannabis. With recent trends toward cannabis legalization, there is concern that increased cannabis use will result in more cannabis-related MVCs.
Delta-9-tetrahydrocannabinol (THC), the primary psychoactive constituent of cannabis, impairs attention, coordination, reaction time and tracking performance.(Desrosiers et al., 2015; Broyd et al., 2016) These effects are most prominent in occasional users whereas frequent users develop partial tolerance to some of these effects.(Broyd et al., 2016; Colizzi and Bhattacharyya, 2018) After smoking marijuana, THC is rapidly absorbed through lungs, resulting in a rapid rise in blood THC concentration with maximum levels within ten minutes. THC then diffuses rapidly into body fat and highly vascularized tissues resulting in a rapid decrease in blood THC concentration.(Grotenhermen, 2003; Huestis, 2007; Odell et al., 2015) In occasional cannabis users, Hartman showed that blood THC levels declined below 5 ng/mL at 3.3 h and below 2 ng/mL at 4.8 h.(Huestis, 2007; Hartman et al., 2016) However, in frequent cannabis users, THC accumulates in body fat from where it is slowly released back into blood resulting in detectable THC in blood long after last use.(Desrosiers et al., 2014) In occasional users, the cognitive effects of THC following acute exposure approximately correlate with blood THC concentrations after an initial lag,(Grotenhermen et al., 2005) however there is poor correlation between THC level and impairment in frequent users.(Desrosiers et al., 2015)
Many jurisdictions have per se limits for THC. Some jurisdictions have zero tolerance laws that make it illegal to drive with any detectable amount of drug in the body. Others establish non-zero per se limits above which it is illegal to drive. However there is concern that these limits will inadvertently criminalize drivers who use cannabis for medical purposes.(Hager, 2017; Valleriani et al., 2017) This systematic review aims to describe blood THC concentrations in frequent cannabis users after over four hours of abstinence.
This protocol is published in the Prospero international registry for systematic reviews (Desapriya et al., 2018).
After data sources were combined and duplicates removed, our search identified 1612 articles. Twenty five manuscripts met criteria for full text review and seven met inclusion criteria. There was a single discrepancy in quality assessment between the two reviewers which was resolved through discussion. An additional eligible publication was found through reference search of the included studies (Fig. 1). Seven of the included articles, representing 5 different studies, were from the same
To our knowledge, this review represents the most current and comprehensive analysis of residual blood THC levels in frequent cannabis users following a period of abstinence. The studies in our review consistently demonstrate that positive blood THC levels, even levels over 2 ng/mL, do not necessarily indicate recent cannabis use in frequent cannabis users. In fact, some frequent cannabis users had THC detected in blood up to 30 days after last use.(Bergamaschi et al., 2013) Five manuscripts
Blood THC >2 ng/mL, and possibly even THC >5 ng/mL, does not necessarily represent recent use of cannabis in frequent cannabis users. Additional research is required to determine the percentage of frequent cannabis users who have prolonged THC elevation during abstinence, and to identify factors associated with prolonged THC elevation. More research into the association between residual THC level and psychomotor performance in frequent cannabis users is required to understand the significance