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Driving Under the Influence of Marijuana

With more and more states legalizing recreational marijuana, it’s easy to overlook the dangers that still exist when consuming the substance. Driving under the influence of marijuana comes with its own unique set of risks motorists should know.

Photo of a driver smoking marijuana while driving

When most people picture impaired drivers, they assume alcohol intoxication is involved. This is the most common form of impairment behind the wheel, but it’s by no means the only substance people abuse when driving.

Marijuana is also a huge concern. It affects both physical and cognitive functions, and drivers impaired by marijuana exhibit slow reaction times and poor judgment. As cannabis is legalized in more states and used more frequently by a variety of people, it’s more important than ever to understand its impact on driving.

The Centers for Disease Control and Prevention (CDC) revealed that 18 percent of Americans used marijuana at least once in 2019. These people must not consume marijuana before getting behind the wheel.

Effects of Marijuana: Is It Dangerous?

The effects of marijuana vary between people and the strain consumed, and it can be difficult to predict how it will impact any given person at any given time. 

In the short term, these effects are common:

  • Dry mouth
  • Dry eyes
  • Headache
  • Dizziness
  • Drowsiness
  • Increased appetite
  • Distraction
  • Increased heart rate

Regular marijuana use can also have many long-term implications, especially if consumed in excess. These may include:

  • Altered brain development in adolescents and young adults
  • Insomnia, which is especially common for daily users
  • Weakened immune system and greater risk of pneumonia
  • Increased blood pressure and, over time, increased risk of heart disease or stroke

Marijuana use disorder is also a cause for concern. This problem occurs when regular users are unable to abstain from consuming cannabis, even when it has a negative impact on their social lives or performance at work or school. Research from JAMA Psychiatry estimates that this disorder occurs for three in 10 users.

Prescription drugs or mental health concerns can exacerbate the complications. For example, individuals with bipolar disorder may find that their symptoms become more severe when they’re under the influence of weed. Additionally, research indicates that people with schizophrenia are more likely to experience psychosis when they use the substance.

Experts from the CDC believe that marijuana smoke can lead to many of the same heart and lung problems commonly associated with cigarette smoke. While further research is required to determine if marijuana smoke leads to an increased risk of death from cardiovascular disease compared with tobacco use, the harmful effects of marijuana smoke are still evident.

All of these potential effects serve to amplify the dangers of mixing marijuana with the operation of a motor vehicle.

Impact of Specific Strains

As the cannabis legalization movement continues to take hold — and medical and recreational consumption increase — dispensaries are eager to release a greater variety of products that will appeal to many consumers.

While different modes of consumption have always produced different effects, the scope continues to increase as the cannabis industry produces new strains. These are sometimes referred to as chemovars (short for “chemical varieties”), as some believe that “strain” is associated with viruses and may have negative connotations.

Some chemovars are specifically designed to increase drowsiness, so using these would be particularly dangerous prior to driving. Most fall into a few main categories, such as:

  • Sativa. Known for its energizing effect, sativa is what many people imagine when they think of “typical” marijuana. This strain contains higher levels of THC (tetrahydrocannabinol) than indica and most hybrid strains.
  • Indica. Thought to produce a “body high,” the indica strain offers a calming alternative to the energizing effect of sativa. It’s frequently used by people who struggle with insomnia and other sleep disorders.
  • Hybrid. As its name implies, this variety involves a blend of the sativa and indica strains. While the effects can vary from one hybrid product to the next, it’s common for hybrid strains to produce feelings of euphoria.
Photo of person preparing marijuana

The cannabis industry increasingly relies on categories such as Type I, Type II, and Type III to reveal the levels of THC and CBD in any of the strains listed above.

Driving and Marijuana

Many people use marijuana without suffering any ill effects. As with alcohol, it’s possible to consume cannabis responsibly and in moderation. Marijuana can even address various health concerns, especially among those who respond poorly to conventional treatment options.

Problems arise when using marijuana before or during activities that require the person’s full energy and attention. Driving under the influence of marijuana is particularly risky.

Drowsiness is arguably the most significant concern regarding marijuana-impaired drivers safely operating a motor vehicle. Falling asleep behind the wheel can lead to a catastrophic accident, as tired drivers will focus more on staying awake rather than being aware and alert motorists.

Driving High Is Always Illegal

Consuming marijuana remains illegal on a federal level, although medical and recreational consumption is permitted in many states. Driving, however, is off-limits after consuming marijuana.

Every state has laws prohibiting the operation of motor vehicles while driving under the influence of marijuana. There is no legal limit for driving high since it’s forbidden to drive after consuming any amount of marijuana.

Complications With Determining Intoxication

While it’s relatively easy to predict how long drivers need to wait after drinking, this is incredibly difficult to determine with marijuana. Alcohol volumes are clearly stated on beverage containers, but THC potency levels can vary depending on the product and how much a person consumes.

Frequency of use also has an impact. In a study published in the journal Traffic Injury Prevention, subjects who claimed to use cannabis on a daily basis tested positive for THC even after abstaining from use for over 12 hours.

Blood tests are the current standard practice for gauging intoxication. These are not reliable, as lab tests indicating THC levels in the blood won’t necessarily reveal the level of impairment related to the drug’s presence.

The unstable nature of THC can also make blood tests less reliable. If blood samples are stored improperly or are not analyzed within three months, THC levels may be underestimated.

This challenge remains relevant as law enforcement shifts from traditional blood tests to more advanced breath tests. The breath-based version draws on nanotechnology to detect THC. Because THC can stay in the body so long after marijuana has been consumed, breath tests aren’t much better than blood samples.

What About Field Sobriety Tests?

Traditionally, law enforcement officials have relied on field sobriety tests (FSTs) to gauge driver impairment. While their accuracy has sometimes been called into question, these tests may provide valuable insight. This is particularly noteworthy in light of the drawbacks of blood and breath tests. Combining FSTs with blood or breath tests may provide the most accurate insights.

FSTs can take many forms, such as:

  • Horizontal gaze nystagmus. This term refers to an involuntary eye motion that is not noticeable to the intoxicated individual. According to a report published in the journal WIREs Forensic Science, this test is unreliable unless the person with a suspected impairment has also consumed alcohol.
  • One leg stand. This test verifies the ability to balance on one leg. The results may reveal heavy drug use.
  • Modified Romberg balance. This unique test reveals time perception issues and physical impairments. Test takers stand in one place with their feet together while tilting their heads back. Subjects must hold this position for thirty seconds.
  • Walk and turn. This is one of the most recognizable field sobriety tests, which determines whether drivers can walk in a straight line.

Recent advancements in testing take the concept of the FST to the next level. Neurocognitive tests promise to reveal whether drivers’ executive function is significantly impaired.

The Massachusetts company DRUID (Driving Under the Influence of Drugs) has created a mobile app that relies on a test of hand-eye coordination. While initially envisioned as a personal assessment app that could prevent intoxicated drivers from getting behind the wheel, the app’s usefulness may extend to law enforcement.

Another test known as AlertMeter may distinguish between everyday consumption and genuine impairment. Proposed as a solution for employee drug testing, AlertMeter may also be helpful for verifying whether drivers are capable of operating vehicles safely if they have detectable levels of THC.

Driving While High vs. Driving Drunk

Driving under the influence is always dangerous, but the specific hazards of marijuana are different from those associated with alcohol. While alcohol impairment tends to prompt aggressive behavior behind the wheel, marijuana users are more likely to become drowsy or distracted.

The actual level of distraction largely depends on how frequently users consume cannabis. Surprising results from the previously mentioned Traffic Injury Prevention study suggest that drivers who regularly use marijuana are more likely to perform well on distracted driving tests “after acute cannabis smoking” than occasional users.

Regular use becomes a lot more dangerous for young users. Research suggests that those who begin consuming cannabis before age 16 are more likely to perform poorly when completing driving tasks in a test setting. Neuroscientist Staci Gruber explains, “Early exposure to cannabis … appears to confer greater difficulty with complex cognitive tasks like driving.”

Finally, drivers’ perceptions of their abilities behind the wheel must be taken into account. Those who consume cannabis are more likely to be aware that they’re impaired and less inclined to speed or make sudden lane changes. Alcohol has the opposite effect, increasing drivers’ confidence even when their reaction time and focus are dangerously compromised.

Often, driving while high versus driving drunk isn’t an either-or situation. As the National Highway Traffic Safety Administration points out, many people drive under the influence of weed and alcohol simultaneously. This can drastically increase the risk of a collision compared to using either substance alone.

Research detailing the exact prevalence of marijuana vs. alcohol-prompted motor vehicle accidents remains limited. A review of law enforcement data published in PLoS One sheds some light on this comparison.

According to this study, drivers are over 17 times as likely to cause fatal collisions if they’re drunk. Those impaired by marijuana are nearly twice as likely to be involved in fatal crashes than their sober counterparts.


No national standard exists to determine how long someone should wait to drive after consuming marijuana. However, experts at the Colorado Department of Public Health and Environment recommend waiting at least six hours after smoking less than 35 milligrams of THC and eight hours after eating or drinking something containing less than 18 milligrams.

For reference, a “typical” marijuana cigarette contains at least 60 milligrams of THC, and most edibles contain around 10 milligrams per serving size. A 12-hour wait is safer, as the high (and subsequent drowsiness) from smoking a typical amount lasts far longer.

Driving under the influence of weed is far too common. A 2019 Morbidity and Mortality Weekly Report study revealed that 4.7 percent of U.S. residents age 16 or older had operated a vehicle after using cannabis. Impaired driving was especially common among males between the ages of 16 and 34.

It is never safe to drive while high. This is different from driving with detectable levels of THC in your bloodstream, which can be safe depending on the amount. Often, marijuana doesn’t disappear from the system entirely for several days or even weeks.

Blood or breath tests can determine marijuana intoxication. Although field sobriety tests occasionally supplement these tests, many field sobriety methods are not reliable for determining marijuana intoxication.

Driving under the influence is always negligent, whether it’s alcohol or marijuana intoxication. Sober driving is the only safe option for operating a motor vehicle.

Alcohol is more likely to make drivers behave aggressively: speeding, tailgating, or changing lanes erratically. Marijuana use can trigger these behaviors, but general distraction, slow reaction times, and drowsiness are more likely.