A recently published two-year study conducted by the Center for Medicinal Cannabis Research at UC San Diego School of Medicine confirms what many of us already knew: that using cannabis, and in particular the psychoactive cannabinoid tetrahydrocannabinol (THC), impairs one’s ability to drive.
However, the study also found that experienced cannabis users’ ability to operate when “under the influence” was not significantly diminished and that there is no correlation.
Study Results
Here’s a basic breakdown of the study:
- 191 regular cannabis users were recruited.
- They were given either cannabis containing 5.9% THC, 13.4% THC, or a placebo.
- Their driving abilities were tested in a simulator, and they were tested over 4 hours.
- 50% of the THC users were impaired, but 50% were not.
- In impaired participants, the sharpest decline in driving was between 30 minutes and 1.5 hours after ingesting cannabis, borderline differences with the placebo group after 3.5 hours, and no differences after 4 hours.
- There was no difference in impairment between those who ingested 5.9% THC and 13.4% THC – both dosed till they felt the same level of “high.”
- Controlled use by experienced cannabis users suggests little-to-no impairment in driving ability, at least for 50% of the drivers in this study.
- The study found no relationship between post-smoking blood THC concentrations and simulator performance.
- There is no correlation between blood concentrations of THC and driving performance, at least on simulators.
What Does This Mean?
This study suggests that while using THC-rich cannabis impairs one’s ability to drive for up to 4 hours, it doesn’t have the same effect on everyone. More experienced users may have built some tolerance to THC, as well as knowing when to stop. With 50% of THC users in this study not being impaired, there is no definite link between THC use and driving impairment. THC blood concentrations post-smoking, especially 4 hours after ingestion, do not determine driving ability.
In the words of one of the researchers:
“The complete lack of correlation between blood concentrations and driving performance was somewhat surprising. It’s strong evidence against developing `per se’ driving under the influence [DUI] statutes.”
What Are Per Se DUI Laws?
A “per se” DUI (driving under the influence) charge is based on a driver having a certain amount of drugs or alcohol in their blood. This study suggests that the level of THC in the blood alone is not enough to suggest impairment, as a high percentage of people showed no impairment.
The researchers suggest that factors such as individual biological differences, personal experience with cannabis, and cannabis administration (ingestion) methods should be considered.
As this would likely drive up the costs of arresting someone for a cannabis-related DUI (i.e., they would have to test for actual impairment and not just THC in the blood), it is unlikely that such an approach will occur. Another thing to remember is that, for a significant amount of people, using cannabis does impair driving ability for up to four hours after ingestion. Other ingestion methods like edibles, which are usually far more potent for most, may impair driving ability to an even greater degree.
However, this shows that it would be unfair to arrest someone for DUI based on blood concentrations of THC alone. Cannabinoids do not work in the same way as alcohol does, and current testing methods do not pick up on what is a rough methodology to test for cannabis-related DUI charges.
I’m An Experienced Driver and Cannabis User – Surely I’ll Be Fine?
Despite the exciting findings of this study, we would never suggest that it’s OK to drive a vehicle or operate heavy machinery under the influence of cannabis and THC.
It is possible to be overconfident about your ability to drive after ingesting cannabis and feel that you’re fine to drive 30 minutes to two hours after consuming cannabis. This is certainly not the case for a good number of people in this study, even the ones who felt they weren’t affected.
What About Driving Under the Influence of CBD?
If you aren’t using THC or using extremely low amounts, then it is unlikely (but still potentially possible) that you’ll test positive for cannabis. Most modern drug tests do not test for cannabidiol (CBD).
CBD isn’t as intoxicating as THC. However, high doses can have sedative effects in some people, so it is wise to avoid operating a vehicle if you have taken over 40 or 50 mg of CBD in one go for at least 3 – 4 hours after ingestion. This is especially true for those not experienced with cannabis and CBD, who may still need to find their ideal dosage.
So Does THC Impair Driving Or Not?
Yes, it does, but those who are experienced and dosed their cannabis carefully seem to show minor impairment, at least in simulations. However, this does not give license to drive after cannabis ingestion, even for the most experienced cannabis users. Remember, these are simulations, and the real world can throw up even more variables when driving safely.
Therefore, we suggest that if you must drive after utilizing your medication, you should wait at least 4 hours to overcome any possible intoxication and impairment. Ideally, we advise not driving at all for the rest of the day after medicating, if you can avoid it! We recommend safe and responsible use of medical cannabis, so steer clear of operating heavy machinery after ingestion. Furthermore, don’t break the law and become another unnecessary cannabis arrest statistic!