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Is it safe to smoke cannabis while driving?

February 23, 2024

Cannabis looks set to become legal in Germany. There will be a THC limit for its use in road traffic, as with alcohol. But setting a limit is difficult.

Cannabis next to a car key
Legal limits on alcohol and cannabis serve as an objective measure to determine whether a person is fit to driveImage: Fabian Sommer/dpa/picture alliance

After months, if not years, of debate, German legislators took the first step to legalize cannabis on February 23. If the law passes final scrutiny in Germany's upper house, it will take effect on April 1.

There will be a legal limit for THC — the psychoactive element in cannabis — for users in road traffic, similar to Germany's blood alcohol limit of 0.5 mg/ml, or a blood alcohol concentration of 0.05%.

The current limit for cannabis in Germany is 1 nanogram (ng) of THC per milliliter of blood. In France, for instance, the limit is half as high as the proposed limit in Germany. While in the Netherlands, the limit for occasional users is three times higher.

Effects of cannabis different from effects of alcohol

Consuming cannabis can influence driving behavior. Cannabis contains psychoactive substances such as tetrahydrocannabinol (THC), which act on the central nervous system and are responsible for its intoxicating effect.

But unlike alcohol, which affects the body relatively quickly, the effects of cannabis can be delayed and last for longer after consumption.

Cannabis leaf on traffic light
Cannabis use affects driving behavior, but the effects can be different from those of alcohol and can last longerImage: Rolf Vennenbernd/dpa/picture alliance

Blood alcohol concentration (BAC) correlates quite reliably with a certain degree of impairment and an increased risk of accidents. But the relationship between the THC concentration measured in the blood and the ability to drive is less clear. There is no explicit threshold above which someone can or cannot drive safely.

This may be why it's difficult to set a uniform limit for cannabis in road traffic that is both scientifically sound and practical to apply.

Why do THC values fluctuate?

When a person smokes weed, the THC concentration in the blood rises sharply at first, but then falls again just as quickly.

This is because THC is distributed in fatty tissue with a good blood supply, like the lungs, heart, liver and brain.

From there, it returns to the bloodstream over days — it can be detected in the blood weeks after consumption.

The way in which cannabis is consumed influences the speed and concentration of THC in the blood. Smoking cannabis leads to a faster absorption of THC and higher concentrations in the blood.

Consuming cannabis in cookies or teas means the THC has to pass through the digestive system before it can be metabolized by the liver and then enter the bloodstream.

It can take anywhere from 30 minutes to several hours for the effects to kick in, but they can also last longer as the body digests THC slowly.

The effects of cannabis on cognitive function, coordination, reaction time and perception can vary greatly from person to person, depending largely on their sensitivity to THC and usual consumption habits.

How does cannabis reduce a person's ability to drive?

Drivers who use cannabis often attract attention because they drive particularly slowly. They notice their limitations themselves and try to compensate.

Cannabis slows reaction times. Drivers may have delayed responses to changing traffic situations, for example, when braking or avoiding obstacles. Cannabis consumption also reduces attention and concentration times. Drivers can overlook important traffic signs, pedestrians or other vehicles.

Amsterdam bans public pot smoking

However, cannabis consumption can also impair judgment. Drivers might make riskier decisions, drive too fast or ignore right-of-way rules.

Motor skills and perception of space and time can be impaired by cannabis use. Drivers might find it more difficult to judge the speed and distance of other vehicles.

What rules apply internationally?

The regulations on cannabis use in road traffic vary considerably from country to country.

Some countries, such as Sweden, have a zero tolerance policy on driving under the influence, whether it's cannabis or alcohol. Any detection of drugs in the blood can lead to criminal penalties, regardless of the quantity of physical and cognitive impairment.

Other countries have set limits for THC content in the blood or saliva. These vary considerably:

  • France: 0.5 ng/ml THC in the blood
  • Switzerland: 1.5 microgram (ng)/l THC in the blood
  • Netherlands: 3 ng/ml THC in the blood

Some countries use combined approaches in which behavioral observations and standardized field tests are carried out alongside fixed limits to assess fitness to drive.

This is the case in the United States, for example. Some states have limits for THC in the blood, while in others, unfitness to drive is based on behavioral indicators.

Why do we need a legal THC limit?

Legal limits in road traffic serve as an objective measure to ensure the safety of all road users, regardless of whether a person subjectively believes they are fit to drive.

This article was originally written in German.

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