THC in Blood and Saliva Not Indicative of Impairment, Studies Say

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Measuring impairment has more to do with the individual user, and much less with their blood content. While the drug driving laws continue to use blood and saliva as measures of impairment, up-to-date studies reveal the inaccuracy of these protocols.

One such study published in Neuroscience & Biobehavioral Reviews revealed that blood and saliva testing for THC has no actual correlation with impairment. Referencing their own studies as well as data from nearly 30 other publications, the research team was able to shed some light on the differences between measuring THC and alcohol in roadside tests, and which factors play a part in assessing impairment.

“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.”

Lead author Dr Danielle McCartney,
from the Lambert Initiative for Cannabinoid Therapeutics

The science tells us that, when it comes to measuring cannabis impairment, several factors play a role. These include dose, mode of ingestion, and length of treatment.

A patient who has a long history with medicinal cannabis was less likely to register positive in roadside tests for THC, even when it is present.

Dose plays a large factor in the effects of medicinal cannabis, and low level prescriptions are still identifiable in roadside tests, even when a driver is unimpaired.

On the other hand, the research found that patients with a less frequent dosage would display a positive THC blood content in roadside tests. Police would interpret this as impaired, but do not take into account whether or not there are true signs of intoxication.

“This suggests that blood and oral fluid THC concentrations are relatively poor indicators of cannabis-THC-induced impairment,” says Dr Danielle McCartney

Despite these results showing that THC in blood has no relation to decreased driving performance, testing for presence remains the chosen mode of policing cannabis in Australia. 

What can measure impairment?

How police will be able to accurately measure impairment and detect intoxication in drivers remains the biggest issue for medicinal cannabis patients and Australia’s road safety. However, the data provides a clear view into a method that is not working and opens the door to more effective opportunities.

“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.”

Professor Iain McGregor, Academic Director of the Lambert Initiative

One possible avenue that can help us better understand the effects of cannabis on driving is technology. 

“Smartphone apps that may help people assess their impairment before driving are currently under development and may also prove useful,” said research and Drive Change ambassador Dr. Thomas Arkell

Ways to support Drive Change:

If the laws wrongly identify medicinal cannabis patients as intoxicated, while some drivers who are intoxicated do not register, then the laws are failing to effectively protect citizens on the road. It also discriminates against the thousands of medicinal cannabis patients who drive responsibly.

A word of caution: the science also tells us that drivers shouldn’t rely on their own judgment of impairment to deem themselves fit to drive. Instead, medicinal cannabis patients should be vocal about their condition, their positive experiences with medicinal cannabis, and how the discriminatory laws affect them. One way of doing so is by writing to local MPs.

We know that reaching out directly is one of the most effective ways to create change, and this is your chance to make it happen.

Explore other meaningful ways to support Drive Change.

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