How Smoking Cannabis Effects Driving

Driving under the influence of cannabis increases the risk of traffic collisions and causes millions in property damage each year. With cannabis use on the rise, it’s important that we understand the relationship between how much cannabis we consume and its effect on our driving ability.

A new study seeks to quantify how different amounts of THC in the blood can affect reaction time and driving performance.

How We Process Cannabis

Cannabis users most frequently consume the plant by smoking its dried flower. Inhaling cannabis smoke quickly leads to a rise in THC levels in the blood. These levels usually peak within 10 minutes of smoking.

From the blood, it becomes absorbed into fat and liver tissue. After 6 to 10 hours, THC is almost entirely gone from the blood. It’s then slowly processed and released from storage in fat and liver cells.

Some evidence suggests that THC may linger in the brain even when it can’t be detected in the blood.

Cannabis and the Brain

Cannabis has well-defined effects on the brain. Consuming cannabis typically results in impaired motor function and cognition and also increases reaction time. Occasional consumers feel these effects most strongly.

Effects on Driving: What We Know

Considering these effects, it’s no surprise that cannabis intoxication can affect your ability to drive. Studies using driving simulators show that after consuming cannabis, people performed worse and took more risks like weaving. Interestingly, those under the influence of cannabis were more likely to drive at a lower speed and give more headway between vehicles than those who had consumed alcohol.

However, driving under the influence of cannabis is still linked to increased accident risk. And unfortunately, people continue to drive after consuming cannabis despite the risk and significant cost in property damage. In 2012, Canada reported a staggering $1 billion in estimated property damage from nearly 8,000 cases of people driving under the influence of cannabis.

Linking Blood Concentrations of THC and Driving Ability: A New Study

One challenge is that, unlike alcohol, we don’t know what the acceptable blood THC threshold is for driving.

Most people know their limits when it comes to alcohol. You can enjoy a drink or two with dinner and still be within the legal limit to drive. But when it comes to cannabis, we’re flying blind.

In a new study published in the journal Clinical Chemistry, French researchers worked to define the relationship between cannabis dose, reaction time, and driving ability. The researchers recruited 30 healthy adult male cannabis users between the ages of 18-34.

They split the study participants into two categories: chronic consumers who smoked 1-2 joints per day and occasional consumers who smoked 1-2 joint per week. Then they had the participants smoke a placebo cigarette or a cannabis cigarette with either 10 or 30 mg of THC (administered on different days of the study).

After smoking, the researchers evaluated the participants for reaction time and driving simulator performance. The researchers also took multiple blood samples from the participants over the course of 24 hours after smoking.

The Results

Blood levels of THC hit a peak twice as high in the chronic consumers than the occasional consumers. Despite a controlled protocol for smoking (“inhalation for 2 seconds every 40 seconds over a 10 minute period, exactly 15 puffs”), the chronic consumers inhaled more of the available THC. THC also stayed in the blood of chronic consumers much longer and was detectable even after 24 hours.

Unsurprisingly, reaction times went up and driving performance went down after cannabis consumption for both groups. The increase in reaction time was dose-dependent—the more cannabis consumed, the longer the reaction time—but driving performance was not.

Even though the chronic consumers had more THC in their blood, their reaction times and driving performance were less affected than the occasional consumers. Occasional consumers also experienced the negative effects for longer than the chronic consumers.

The Implications

This study confirms one finding that we already knew—cannabis increases reaction time and decreases driving performance.

However, it lends more evidence to another theory that scientists still debate—chronic cannabis users become more immune to the impairments of cannabis intoxication than novice users. While the study included occasional and chronic cannabis consumers, I wish they had included non-cannabis or novice consumers (perhaps a group of people who have used cannabis fewer than 5-10 times in their life). It would have been interesting to see the effect on reaction times and driving performance (would it be even worse?) in this group.

Unfortunately, these results don’t lead to a clear cut recommendation for safe blood THC levels while driving. Ironically, the participants with the highest blood levels of THC (the chronic consumers) performed better.

Closing Thoughts

This study brings us closer to understanding the relationship between the amount of THC ingested and driving ability.

Interestingly, THC blood levels aren’t necessarily correlated with better reaction times and driving performance. While driving under the influence is never a good choice, more experience cannabis consumers will likely perform better than novice users even when they have higher THC levels in their blood.

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