Marijuana and driving: How much is too much?

Two U.S. states, Colorado and Washington, have recently legalized the recreational use of Marijuana and both are struggling with the question of how much is too much when it comes to Marijuana and driving.  Colorado and Washington have both recently passed legal limits for tetrahydrocannabinol or THC in the blood at 5 ng/ml but Washington’s is a “per se” law while Colorado’s is not.  A per se law sets an intoxication standard based purely on the concentration in the blood.  So in Colorado, even if a driver is above the 5 ng/ml limit they have the opportunity to present evidence that they are not impaired.

Some marijuana advocates argue that the 5 ng/ml standard is too low and that some medical marijuana users are always above that level.  What is the right level?

Below is a very unscientific examination of the issue by a local television station in Washington State.

There doesn’t appear to be a scientific consensus on what the maximum THC blood level should be.  The National Highway Traffic Safety Administration in the U.S. is researching the issue with results expected next year.  Until more research becomes available the debate continues.

  • Duncan Vernon

    Readers may be interested that the government is creating a new offence of drug-driving in Britain, and established an expert panel to consider appropriate limits.

    The panel recommended that if limits were used then a 5 μg/L threshold limit in blood for THC, or a 3 μg/L limit where more than 20mg of alcohol is present in 100ml of blood would be appropriate.

    Limits for other drugs were also recommended.

    The final report giving the recommendations – and the process used – can be accessed here: