Last updated February 21, 2018 at 10:07 am
Questions are being asked about how we test drivers for the effects of weed – and there are no easy answers.
As more and more places legalise marijuana, there are worries about the effect it could have on road safety.
Just like getting blotto on alcohol affects your ability to drive, researchers are concerned about the effect of being high behind the wheel.
However, no roadside test for marijuana intoxication actually exists. And that is a issue that has been made urgent for Californian lawmakers, where recreational use of marijuana is now legal.
But determining whether a driver is safe behind the wheel or not is impossible.
And that, toxicologists say, is a problem.
Pot is different from alcohol
Alcohol can impair a user more than cannabis, and we know that the risk of an accident while driving increases in proportion with blood alcohol concentrations.
This allows a blood alcohol concentration to be a valuable proxy for intoxication.
But pot is different: there’s lots of things that can affect how impaired someone is at a concentration of THC. For example, whether it is smoked or eaten can change how much it affects someone.
And having cannabis and alcohol at the same time makes the high higher, and the alcohol buzz longer.
“There is no one blood or oral fluid concentration that can differentiate impaired and not impaired,” says Marilyn Huestis, who has spent more than 20 years leading cannabinoid research.
“It’s not like we need to say, ‘Oh, let’s do some more research and give you an answer.’ We already know. We’ve done the research.”
Another problem is that THC is quickly flushed from your blood.
Positive tests 30 days after last THC use
An occasional user can be impaired for six to eight hours, but their blood THC concentrations can drop to effectively zero after 2.5 hours.
If there is a delay in taking blood to test, the results could be completely worthless.
“If someone is driving impaired, by the time you get their blood sample, you’ve lost 90% or more of the drug. So, we have to change what we do at the roadside,” says Huestis.
On the other hand, long-term users have the opposite problem.
THC accumulates in the tissues of long-term users and then slowly releases over time, meaning that chronic users can test positive 30 days after last using.
Even the impairment of your motor skills can last for three weeks.
Toxicologists don’t support legal limit
For these reasons, a lot of toxicologists don’t support a legal limit for driving like there is for alcohol.
It may come down to requiring police officers to make a judgement, but that just opens up a whole range of problems around prejudice.
There is also research into trying to find other markers of marijuana use and impairment, including saliva or breath tests.
That research could be important for more than just driving. These new tests could also be used to help treat drug dependence, and it could also be really useful for medical marijuana uses where the therapeutic dose needs to be monitored.
“You want people to be taking medicinal cannabinoids and now you know that their driving is going to be impacted,” says Huestis. “So how do you handle that problem?”
The paper has been published in Trends in Molecular Medicine