Measuring Impairment

Knowing when it’s safe to drive.

Studies on driving performance and concentrations in blood and saliva of delta-9-tetrahydrocannabinol (THC) indicate they’re poor or inconsistent for measuring impairment.

  • Higher blood THC concentrations are weakly associated with greater impairment in occasional users, while no significance was detected in regular consumers.
  • Researchers found ‘subjective intoxication’ – how ‘stoned’ individuals reported they felt – is only weakly associated with impairment.
  • Drivers shouldn’t just rely on their own perception to decide if they are safe to drive – better to wait at least three hours depending on the dose, before performing safety-sensitive tasks.
  • Inexperienced consumers can ingest a large oral dose of THC and be unfit to drive, yet register low THC concentrations.
  • Conversely, an experienced consumer, might smoke a joint, show very high THC concentrations, but show little if any impairment.
  • This is a problem for the rising number of patients who are using legal Cannabis for medical purposes yet are prohibited from driving.

There is currently no standardised definition of impairment associated with medical cannabis use and no general consensus on how to measure or define this impairment. Unlike with alcohol, the relationship between cannabinoid and neurocognitive or functional impairment remains undetermined. While evidence supports a positive relationship between THC dose and impairment, an accurate blood concentration range has not been determined

Some studies suggested THC blood concentrations between 2 and 5 ng/ml are associated with impairment but these measures do not consistently correlate with impairment across individuals, likely due to the complex nature of THC pharmacokinetics and metabolism, strongly impacted by individual factors such as genetics and tolerance to THC. 

There is evidence that medical cannabis patients who use THC regularly develop tolerance to many of the impairing effects of THC. Tolerance has also been found with recreational cannabis use, with experimental studies demonstrating that frequent recreational cannabis users, with use more than four times per week, developed psychological and behavioural tolerance, and showed no significant impairment in neurocognitive function or motor side effects compared to infrequent users at the same dose of THC.

Other research demonstrates that tolerance is incomplete, and people who use cannabis regularly still demonstrate some impairment, albeit blunted, after acute use. Determining the duration of potential THC impairment, and what THC dose a medical cannabis patient should take to minimise neurocognitive impairment, proves to be challenging.

There are some unique considerations when studying impairment in medical cannabis patients, defined as someone who uses cannabis under the guidance of a medical practitioner, compared to recreational cannabis users.

Medical cannabis patients often use THC to manage symptoms for a variety of conditions including chronic pain, insomnia, PTSD, autoimmune conditions and neurological disorders, that induce a certain level of neurocognitive impairment by themselves. By treating these symptoms, their neurocognitive and psychomotor functioning may actually improve.

Medical cannabis patients also have different patterns of use, including a more consistent and standardised dosing schedule. They often consume cannabis orally, which lengthens the time until onset and the duration of effect after use, and many choose to use chemovars high in cannabidiol (CBD), which is non-impairing.

If medical cannabis patients are starting THC, most start with low-dose THC products, with doses titrated to obtain symptomatic relief while purposely avoiding impairing side-effects.

Source: Duration of Neurocognitive Impairment With Medical Cannabis Use: A Scoping Review 


Saliva – poor measure of Cannabis impairment


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