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Marijuana and Driving: What You Should Know
cannabis leaf and car key

Studies show it is not safe to drive while high. You should wait 4 hours after smoking before driving.

Driving high is a form of impaired driving. Your ability to drive safely can be significantly affected after using marijuana.

Impaired driving is the leading cause of both injury and death in North America. In Canada, there is a drug-impaired driving offence every 3 hours.

Many people believe using marijuana or other drugs isn’t as bad as driving drunk. But in 2010, almost as many drivers died in road crashes after using drugs (34%) as those caused by drinking and driving (39%).

So before you get behind the wheel or get into a car with another driver, it’s important to understand the effects of marijuana on driving.

Is It Safe To Drive While High?

Is It Safe To Drive While High?
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It is not safe to drive while high. Marijuana can affect your ability to react, making it unsafe to drive and increasing your chances of a crash.

Some reports show that people who drive within 1 hour of using marijuana may be almost twice as likely to be involved in a car accident compared to those who don’t consume marijuana.

How long should you wait before driving?

It is recommended that you don’t drive for at least 4 hours after inhalation, 6 hours after oral ingestion and up to 8 hours if you are experiencing a sense of “euphoria” or “high” after inhaling or ingesting marijuana.

How common is driving high?

Drivers’ perceptions of the risks of marijuana use and driving have steadily declined over the last decade, perhaps because of the change in laws and public debate over the acceptance of marijuana.

Many drivers have admitted to driving after using marijuana just 1 hour after intake or were passengers in a vehicle driven by someone who had used marijuana.

What are the risks of driving high?

Marijuana users are 2 to 6 times more likely to crash than drivers who are not impaired. Reasons not to drive while high include:

  • Risk of being involved in a car crash
  • Hurting or killing yourself, someone you care about, or innocent strangers
  • Getting arrested, paying a fine and having your license suspended
  • Being convicted with a criminal record

Does marijuana increase the risk of an accident?

Studies have shown that THC can impair your ability to drive. Marijuana is the drug most often found in drivers involved in fatal car accidents.

However, the role of marijuana in these accidents is not always clear, because it can be detected in body fluids days or even weeks after initial intoxication and because it can be combined with alcohol.

What are the dangers of mixing marijuana and alcohol?

One study looked at how driving skills are affected by marijuana or alcohol, and their combined effects on driving. Subjects were given various doses of marijuana with or without alcohol and tested in normal traffic.

When low doses of THC were given, there was a moderate impairment. But when THC was combined with even a low dose of alcohol — a common scenario at parties —  driving performance was severely affected.

How Do I Know If I’m Impaired?

How Do I Know If I'm Impaired?
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Marijuana impairs the ability to drive by affecting your coordination, attention, judgement, reaction time and decision-making skills.

Most studies show that using marijuana impairs your mental functioning when it comes to “cognitive” or thinking ability and short-term memory tasks.

Signs of marijuana impairment include:

  • Red or bloodshot eyes
  • Delayed reaction time
  • Distorted sense of your surroundings
  • Poor coordination
  • Feeling anxious or panicky
  • Feeling dizzy or tired

What should I do if I’m impaired?

If you are impaired, don’t get behind the wheel. If somebody you know is impaired, refuse to drive with them. Try to find something to do until the high wears off (approximately 4 hours after smoking).

Being under the influence of marijuana means that your ability to react appropriately in any given situation while driving may be significantly affected. A split-second decision could mean the difference between life and death.

Does marijuana always cause driving impairment?

Some people erroneously believe that marijuana makes you a better driver, because you’re more cautious. Even if you feel like you are being cautious, your reaction time and other driving skills are impaired by marijuana.

Some people may also believe that it is safe to drive while high if you have a tolerance. But don’t be fooled. Marijuana can still make it dangerous to drive even if you are a seasoned user.

How Is Marijuana Detected in Drivers?

How Is Marijuana Detected in Drivers?
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Marijuana-impaired driving is something you can be charged with, but law enforcement officers often have problems detecting whether or not a driver is under the influence.

Right now, police use roadside behavioral testing for marijuana use. They may also run a blood or urine test to verify that marijuana was taken, but these tests have poor time resolution and can’t always indicate exactly when the driver was impaired.

Is there a marijuana breathalyzer?

There is currently no “breathalyzer”-type test for marijuana as there is for alcohol. However, there are several devices that are currently under development, including the use of a roadside saliva swab.

What happens when you are pulled over?

If you are pulled over, police will do a visual inspection, and may see if they can detect the scent of marijuana in the car.

If police suspect that you are high, you can be asked to do a standardized sobriety test on site or at the police station. These tests usually consist of an evaluation of your eyes (red or dilated) and coordination challenges. Police can also request a drug test.

What types of marijuana drug tests can be used?

Urine tests can pick up traces of THC several days or more after smoking marijuana, and for heavy users, traces may even be found weeks after they have stopped using it. The cut-off for urine tests is 50 ng/mL.

Blood tests may be requested in the case of a roadside accident or to check for intoxication. A positive test can show up just a few hours after using marijuana, and regular users may test positive even a day or two later. In fact, traces of THC can be detected by standard blood tests for up to 7 days in heavy users.

Saliva tests are a newer method that is currently being developed as a roadside test for marijuana use. Saliva tests can detect whether marijuana was used within 24 hours. This is because the linings of the mouth absorb THC and release it into the saliva after marijuana use. The time resolution of 12-24 hours is much better than that of urine or blood tests.

What Is The Legal Limit?

What Is The Legal Limit?
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United States

US laws vary depending on the state, but states that have legalized marijuana often have regulations when it comes to driving under the influence.

Some areas have “zero tolerance” laws, which make it illegal to drive with any level of THC in the blood. Other states have set limits for levels of THC in the blood.

Washington has set legal limits on THC levels for drivers. There, drivers can have no more than 5 nanograms of THC per milliliter of blood. Colorado has similar laws, also setting the limit at 5 ng/mL.


In April 2017, the Government of Canada revised their impaired driving laws to include stronger legislation and more severe punishment for those who drive under the influence of drugs, including marijuana.

Canada’s new regulations would be gauged according to THC levels per milliliter (mL) of blood:

  • 2-5 ng/mL within 2 hours of driving could have a maximum fine of up to $1,000
  • 5 ng/mL or more within 2 hours of driving would be considered an offence and you could be prosecuted
  • THC and alcohol: A combination of more than 2.5 ng/mL of THC and 50 mg/100mL of blood alcohol within 2 hours of driving would be a chargeable offence.

Challenges of Legal Limits

While scientists agree that THC impairs driving performance, levels of THC may not always be a reliable indicator of the actual degree of impairment.

Zero level legislation may unfairly convict those who are heavy or regular marijuana users and who have a higher tolerance level and are not impaired at the time of driving.

Heavy marijuana users may have THC levels that remain in the brain and body, indicating a chronic level of impairment. And some users may not show the obvious signs of impairment even though they have significant THC concentrations in their blood.