The Facts About Driving After Marijuana Use

Technology to test for marijuana impairment exists but the instruments are not small enough to be used on the road like with an alcohol breathalyzer test, says Dr. Yifrah Kaminer of UConn Health.

A police traffic stop. (Getty Images)

A police traffic stop. (Getty Images)

As the national debate on marijuana legalization continues, we asked Dr. Yifrah Kaminer, a professor of psychiatry and pediatrics in the Department of Psychiatry and Alcohol Research Center at UConn Health, to share his assessment of the impacts of marijuana use – particularly related to driving.

Q. How does marijuana use affect a person’s cognitive abilities?

A. Marijuana use causes impairments in attention, concentration, decision making, impulsivity, and working memory. In daily users, this impairment may last for up to four weeks after cessation of use. It was reported that adolescent-onset long-term use of at least four days per week predicts an eight point decline in IQ on average. This finding was not reversible with a year of abstinence. Frequency of marijuana use predicts decreased executive functioning and learning, especially for adolescents who initiate use by age 14.

Q. Are the effects from marijuana use similar to those from drinking alcohol?

A. Similarly to alcohol, cannabis alters perception and psychomotor performance, which may contribute to an increased risk for car crashes. In one study of cannabis users in Colorado, where recreational marijuana is legal, more than 50 percent of cannabis users believed that it was safe to drive under the influence. The researchers found that traffic fatalities of drivers testing positive for cannabis increased steadily from 2006. The percentage of total fatalities of drivers testing positive for cannabis increased by approximately 90 percent since the legalization of cannabis for recreational use in 2013 through 2017.

Q. Is there a certain amount or level that has to be used for someone to be considered impaired?

A. Although the legal limit is 5 ng/ml, in many fatalities the level was 30 or more ng/ml. There is a personal variability that novice users get ‘impaired’ under lower levels than chronic/high-frequency, high-dosage users who develop a tolerance.

The consensus is do not drive less than three hours after use. If you’re involved in a crash, the fact that you have a cannabis medical card won’t be accepted as an excuse.

Q. How would law enforcement officers test for this impairment?

A. This difficult work is in progress. The technology exists but the instruments are not small enough to be used on the road, like with an alcohol breathalyzer test or TSA explosive detectors.

Q. Does marijuana use increase the likelihood of experimenting with other drugs, potentially increasing the possibility of impairment?

A. About one in six youth who use marijuana develop a cannabis use disorder within 12 months of first use, as compared to one in 11 among adults. Adolescent marijuana use also predicts a two- to three-fold increased risk of using other substances. A recent independent study conducted in Connecticut concluded that there is a relationship between marijuana use and the abuse of opioid pain medications and heroin. This study, which included a control group, is based on data from a total of 6,000 Connecticut high-school students over a period of 12 years. Marijuana users reported 14-fold abuse of pain medications, and more than four-fold of heroin use respectively compared with non-marijuana users.