Research by the Lambert Initiative has found THC concentrations in blood and saliva are poor indicators of cannabis-induced impairment, raising questions about current drug-testing methods.

The findings contrast with the much stronger relationship between blood alcohol concentrations and driving impairment, and have implications for the application of drug-driving laws globally, according to the researchers.

The team pooled data from 28 publications involving consumption of either ingested or inhaled forms of cannabis and characterised the relationship between blood and oral fluid THC concentrations and driving performance. 

Dr Danielle McCartney

For infrequent or occasional cannabis users, some significant correlations between blood and oral fluid THC concentrations and impairment were observed. However, the researchers noted most of these relationships were “weak” in strength.

For regular (weekly or more) cannabis users, no significant relationship between blood THC concentration and driving performance was observed.

The findings raise questions around the validity of current methods used to assess cannabis-related impairment, such as widespread random mobile drug-testing for THC in saliva.

Lead author Dr Danielle McCartney said: “Higher blood THC concentrations were only weakly associated with increased impairment in occasional cannabis users, while no significant relationship was detected in regular cannabis users.

“This suggests blood and oral fluid THC concentrations are relatively poor indicators of THC-induced impairment.”

“Our results indicate that unimpaired individuals could mistakenly be identified as cannabis-intoxicated when THC limits are imposed by the law. Likewise, drivers who are impaired immediately following cannabis use may not register as such.”

“Of course, this does not suggest there is no relationship between THC intoxication and driving impairment,” Dr McCartney added. “It is showing us that using THC concentration in blood and saliva are inconsistent markers for such intoxication.”

The study, published in Neuroscience & Biobehavioral Reviews, also found that subjective intoxication — how “stoned” an individual feels — is only weakly associated with actual impairment. 

Co-author Dr Thomas Arkell warned: “Individuals are better to wait between three and 10 hours following cannabis use, depending on the dose and route of administration, before performing safety-sensitive tasks.

“Smartphone apps that may help people assess their impairment before driving are currently under development and may also prove useful.” 

Lambert Initiative
Lambert Initiative academic director Professor Iain McGregor

Lambert Initiative academic director Professor Iain McGregor added: “THC concentrations in the body clearly have a very complex relationship with intoxication. The strong and direct relationship between blood-alcohol concentrations and impaired driving encourages people to think that such relationships apply to all drugs, but this is certainly not the case with cannabis.

“A cannabis-inexperienced person can ingest a large oral dose of THC and be completely unfit to drive, yet register extremely low blood and oral fluid THC concentrations. On the other hand, an experienced cannabis user might smoke a joint, show very high THC concentrations, but show little if any impairment.  

“We clearly need more reliable ways of identifying cannabis impairment on the roads and the workplace. This is a particularly pressing problem for the rapidly increasing number of patients in Australia who are using legal medicinal cannabis yet are prohibited from driving.”

Hannah Adler

Hannah is a communications professional and early-career researcher in the disciplines of health communication and health sociology. She is a PhD student at Griffith University currently writing a...