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Researchers say the THC component of cannabis can produce mild driving impairment for up to 4 hours. Westend61/Getty Images
  • Researchers say CBD without THC doesn’t appear to affect driving abilities, but the THC may impair driving for up to 4 hours.
  • Outside experts question the study, saying there were only 26 participants and the doses given to them were lower than what an average person would use.
  • Experts also point out that driving laws are different in each state, so you should be aware of what the standards and punishments are in your area.

In a new study, researchers say cannabidiol (CBD) without the intoxicating chemical tetrahydrocannabinol (THC) doesn’t impair driving.

However, the researchers say that THC itself can produce mild driving impairment that can last up to 4 hours.

Researchers from the Lambert Initiative for Cannabinoid Therapeutics at the University of Sydney say it’s the first study showing that CBD lacking its active intoxicant doesn’t affect a person’s ability to safely navigate a vehicle.

“That’s great news for those using or considering treatment using CBD-based products,” said Thomas Arkell, a PhD candidate at the University of Sydney and a lead researcher on the study, in a press statement.

In recent years, millions have turned to cannabis and CBD for health-based reasons. Doctors are using it to treat epilepsy and relieve symptoms of chronic pain as well as to ease anxiety and depression.

This increase comes despite recent concerns that the potency of cannabis may be increasing along with potential addiction issues.

“There was no evidence to suggest that CBD would pose a driving risk, but the study likely included it because it had not yet been actually studied,” Dr. Jordan Tishler, a Harvard-trained cannabis specialist and chief executive officer of inhaleMD, told Healthline.

“There is a substantial body of literature on the driving risk associated with THC use. However, the results of that literature are entirely mixed, showing anywhere from no excess risk to moderate excess risk.”

“For comparison, some studies have shown the cannabis driving risk to be about two times being sober, but alcohol creates eight times the risk,” he noted.

The new study involved giving 26 healthy subjects four different types of vaporized cannabis in random order on four different occasions.

They were also given different mixes of CBD and THC.

Each participant’s driving performance was then assessed in real-world conditions, along a 100-kilometer stretch of public highway in a dual control car with a driving instructor. They were tested from 40 minutes to 4 hours after ingestion.

The tests were conducted at Maastricht University in the Netherlands, using a test measuring the standard deviation of vehicle position.

Researchers said that while previous studies looked at the effects of cannabis on driving, most focused on smoked cannabis containing only THC and didn’t look at the duration of impairment.

“This is a groundbreaking study in many ways, but has its limitations,” Dr. Pritesh Kumar, the director of the Switzerland-based cannabis consulting firm Phytosciences, told Healthline. “The primary one being the small number of subjects. However, the effect size for CBD-dominant cannabis may not have excluded clinically important impairment, and the doses tested may not represent common usage.”

“Most consumers of CBD, on average, consume greater than 13.75 mg of CBD (the dose used in the study) in one serving, so it may not necessarily be significant from that perspective,” he added.

Kumar also said methods matter when it comes to whether someone smokes, vapes (which was done for the study), or ingests cannabis.

“In comparison to smoking and inhalation, after oral ingestion, systemic absorption is relatively slow resulting in maximum THC plasma concentration within 1 to 2 hours, which could be delayed a few hours in certain cases. The effects of oral cannabis can be much longer in duration than inhaled or vaporized cannabis,” Kumar said.

Tishler said the biggest point taken from the study might not be CBD.

“The real story here is the THC side,” he said. “[The study] showed that, one, after 4 hours the risk associated with the THC was gone. Two, that the risk associated with THC use during the initial phase was between 0.8 to 3.8 cm deviation. While this may be statistically significant compared with placebo, this is unlikely to represent any real-world problem. In most driving situations, vehicle deviation of 2 inches of so would not be the difference between a fatal crash or not.”

Tishler said using 26 subjects was enough to reach statistical significance, but not in real world circumstances.

“The popular answer seems to be that [the study is] further evidence that CBD is a wonder drug with no risks,” he said. “In reality, this study does not look at CBD at reasonable doses, nor is there really any reason to suspect CBD to cause driving risk. CBD has other much more significant risks, like drug interactions. However, these results will be used to promote CBD sales.”

Laws are still muddy over when someone is under the influence of THC when driving.

Michelle Donovan, who advises cannabis companies for the legal firm Clark Hill in Detroit, told Healthline that each state determines what is “over the limit.”

“Each individual metabolizes THC at different times,” Donovan said. “For example, a patient who uses medical marijuana regularly may always have THC in their blood, but they may not be ‘high’ at the time they are tested.”

“Each state enacts their own laws to address ‘over the limit’ for driving offenses,” she said. “The offenses can vary from total driving privileges being revoked to prison time. If you do plan on purchasing and using, do your homework first and find out what, if any, ramifications should you be pulled over.”