In a survey of 315 college freshmen from two state universities, one in five students reported using marijuana during the past month and more than half of the male students and more than one-third of the females said they rode in a car with someone who had used the drug. Nearly 44 percent of the boys and 9 percent of the girls said they drove after using marijuana, according to a new study published in JAMA Pediatrics.
In contrast, the study found that a far lower percentage of students reported driving with someone who was drinking -- or driving after drinking themselves. Only 11 percent of male students and 3 percent of female students admitted to driving under the influence of alcohol.
"These findings point to a need for increased efforts to help youth understand that driving after marijuana use is risky," said study leader Jennifer Whitehill, an assistant professor of health policy and management at the University of Massachusetts Amherst, "and they should plan a designated substance-free driver or alternate transportation."
Unlike drunk driving, there is no device like a breathalyzer to test for marijuana use. To test whether drivers are high, police conduct field tests such as having a driver walk nine heel-to-toe steps or stand on one leg for 30 seconds. But a 2012 study suggests that only 30 percent of drivers who have smoked marijuana fail such a field test, compared to nearly 90 percent of drunk drivers.
"We need better methods to measure whether someone is intoxicated with marijuana," said Dr. Nora Volkow, director of the National Institute on Drug Abuse.
In the meantime, Colorado has seen a recent doubling in fatal car crashes involving marijuana use since legalizing the drug for medical use in 2000. While the study couldn't prove that the drug was responsible for the crashes, it raises concerns that fatalities will increase in the state now that pot is legal for recreational use in adults over age 21.
"There's a perception that marijuana isn't harmful," Volkow said, "whereas there have been very effective education campaigns to make people aware that alcohol leads to car accidents."