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    Marijuana and Driving

    How risky is it?

    You may be worried that your loved one will either drive under the influence of marijuana or get into a car with someone who has recently used marijuana. Your concerns are valid.

    Research indicates that many drivers using marijuana are impaired and worse, may not recognize that they are impaired. Talking to loved ones about safety when it comes to marijuana and driving is as important as talking to them about drinking and driving.

    Factors to consider

    There are many factors to consider when it comes to marijuana and driving. Here are a few that can contribute to impairment:

    • Using higher potency and larger amounts of marijuana
    • Having a low tolerance to marijuana or inexperience with marijuana
    • Consuming it by smoking or vaping creating a faster “high”
    • Combining it with other substances like alcohol or opioids
    • Being a relatively new driver with little experience
    • Not letting enough time pass after using marijuana

    It’s important to note that two similar people can use the same amount and potency of marijuana with different results. One person may be impaired while the other person isn’t based upon their experience as drivers and the way their bodies process the drug.

    Signs of impairment

    A person driving after using marijuana can find it difficult to react quickly to make split-second decisions. These decisions include when to brake, speed up, or swerve to avoid a crash.  They may have problems with divided attention. This means it’s more challenging to handle road conditions, the weather, speed, other nearby drivers, etc., at the same time. They may follow too closely or maintain an improper speed. They are also more likely to weave into another lane which may catch another driver off guard and lead to an accident. The risk of drowsiness is also higher when using marijuana and driving.

    Do drivers know they are impaired?

    In a study reported in The Journal of the American Medical Association1, almost 200 drivers who were regular marijuana consumers were divided into 3 groups. One group wasn’t given any marijuana while the other two were given different strengths of marijuana to smoke. All 3 groups were studied while driving in a simulator that measured their driving ability. At specific times, the drivers were asked:

    • “How high are you?”;
    • “How impaired are you to drive?”; and,
    • “Would you drive in your current state?”

    The drivers who smoked marijuana reported being reluctant to drive right after smoking. That said, almost 70% of them thought they were ok to drive after 1.5 hours, even though there was no improvement in their driving ability as measured by the tests. On average, it took 4.5 hours for them to be able to drive safely.

    How do states determine impairment?

    State laws vary in terms of defining impairment:

    • Zero tolerance laws mean no amount of THC, the ingredient in marijuana that produces a “high”, can be in a person’s system.

    • Per se laws prohibit driving if a person’s THC levels exceeds a certain limit (e.g., 2-5 nanograms of THC/millimeter of blood).

    • Permissible inference laws allow a person the ability to prove why they were not impaired in court even if they exceeded the legal limit.

    • Under the influence of drugs (DUID) requires a person to be under the influence of or affected by THC.

    At present, it is easier and less costly to identify impairment for alcohol with a Blood Alcohol Content (BAC) test, but there is no equivalent test for marijuana. In many states, a police officer who pulls over a person for erratic driving will use a breathalyzer first. Often additional testing is not conducted if the BAC test is positive. This means that if the person was using alcohol and marijuana, which is not unusual, marijuana use will be under-reported.

    If the BAC test is negative, the officer may call in a specially-trained drug recognition expert (DRE). The DRE will conduct a field sobriety test and ask for a drug screen (e.g., saliva, urine or blood) which can be used to support a charge of impairment. By the time a person has the screen, their body may have further processed the drug. If this is the case, the results won’t reflect the level of intoxication experienced when the person was first pulled over.

    States like Alabama and Michigan are using roadside oral fluid tests coupled with field sobriety tests to determine if a person is impaired. The oral fluid tests provide results from an oral swab in minutes. They can detect use of marijuana, opioids, methamphetamine, cocaine, benzodiazepines, and other substances. Coupled with information from the officer who pulled the driver over and other tests (e.g., balance, pupil size, pulse rate, muscle tone, etc.), a positive oral fluid test can be used to support an arrest.

    When is it safe to drive after using marijuana?

    The amount of time needed to recover from its use varies from person to person.  Suggested wait times before driving or riding a bicycle are to:

    • Wait at least 6 hours after smoking up to 35 mg of THC. If a person uses more than 35mg, wait longer.
    • Wait at least 8 hours after eating or drinking up to 18 mg of THC. If a person uses more than 18 mg, wait longer.

    Making other plans for transportation may be the best course of action.

    What can families do to protect loved ones?

    It can help to include marijuana along with alcohol in any discussions with teens about safe driving.  Asking questions like “What would you do if the person offering you a ride has used marijuana?” or “What do you think the risks are of driving after consuming edibles or smoking?” can get the conversation started.

    State your expectations around marijuana and driving use with the teen or young adult licensed drivers in your home.  It may help to ask them why anyone would consider driving impaired when they could risk an accident, injury or fatality, increased insurance costs, legal fees, jail time, etc. Let them know ahead of time what the consequences will be for driving under the influence.  Be sure to follow through with any consequences you state.

    Consider asking a loved one of legal age using marijuana how long they think they should wait before driving. It may be better to use a designated driver, public transportation or ride-sharing services if they are unwilling or unable to wait the recommended amount of time.


    1 Marcotte TD, Umlauf A, Grelotti DJ, et al. Driving Performance and Cannabis Users’ Perception of Safety: A Randomized Clinical Trial. JAMA Psychiatry. 2022;79(3):201-209. doi:10.1001/jamapsychiatry.2021.4037