Michelle Dickinson: How will we measure cannabis intoxication if legalised?

New laws could allow people to buy, possess and consume cannabis from licensed outlets. Photo / Jason Oxenham

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The number of countries that have changed their laws around marijuana for medicinal and recreational use is growing. New Zealand is about to enter this debate with a referendum on the legalisation of the drug.

While comparisons are made between how new cannabis rules could follow those already created for alcohol, the truth is that, unlike alcohol, there is no simple way to measure the amount of cannabis in a person's system. So, if cannabis is legalised in the near future - what are the things that researchers need to do to help maintain safety on our roads and workplaces?

The upcoming Cannabis Legalisation and Control Bill referendum proposes legalisation around the production, sale and possession of cannabis. This could lead to new laws allowing people to buy, possess and consume cannabis from licensed outlets.

As we think about how this may affect those who wish to use the drug as well as those that may be affected by the impairment of those taking the drug, many will look towards laws we already have in place. Alcohol, for example, is a controlled drug with clear rules around age, sale and levels allowed in the body while operating a vehicle. The challenge is that there is no easy way to measure the amount of cannabis in a person's system or guidance around how much they can consume and safely drive or operate machinery at work.

Ideally, a cannabis breathalyser would be added to the existing alcohol model for police and home use. Scientists have been trying to engineer one for years, but sadly without success to date. To be useful, a cannabis breathalyser would need to measure the psychoactive ingredient in cannabis, delta-9-tetrahydrocannabinol, or THC as it's more commonly known.

THC levels and driving have been the subject of many research papers, and studies have found trends showing marijuana use causes impairment on every measure of safe driving, including motor-coordination, visual function and completion of complex tasks. The research also shows that the greater the amount of THC consumed, the higher the level of impairment measured.

While alcohol consumption can be measured by being in a fast-absorbing liquid with a known percentage of alcohol, it's difficult to know how big a breath of cannabis may have been inhaled or how efficient the digestive absorption process is when eaten. Because of this, it's almost impossible to know how much THC has been absorbed into the bloodstream.

Roadside alcohol breathalysers are usually based on an electrochemical platinum fuel cell that produces an electrical current that oxidises alcohol molecules in the breath. The more alcohol that is oxidised, the greater the current.

Unlike alcohol, there is usually only a tiny presence of THC molecules in the breath, which makes it a difficult molecule to detect chemically.

This week new research in the journal Organic Letters found a new way to measure THC that shows promise for a different way to detect THC. Using oxidation by pumping electricity into the THC molecules, it converts a phenol group into a quinone that absorbs light at a different wavelength and therefore can be detected. While still in the lab-bench phase, this discovery might move the world one step closer to being able to measure THC concentrations in the body.

There is still a lot of work to do in this space as there are still no clear quantified THC and cannabis intoxication levels that correlate to physical impairment. This new research, however, may bring us one step closer to real-time measurement and further experiments to help provide guidance around THC levels and workplace and driver safety.