Authors: Robert L. DuPont, MD, and Mark S. Gold, MD

Everyone knows alcohol and driving don’t mix, but what about drugs and driving? Few people know that it is illegal to drive under the influence of a drug, including prescription drugs, some over-the-counter drugs, marijuana, and other illicit drugs.

Drugged driving is a major highway threat. In 2007 aboutone in six (16%) of weekend nighttime drivers tested positive for a potentially impairing drug. Half of those (8%) were positive for marijuana.

Drugged drivers put themselves and others on the road at serious risk. The presence of a drug in a drivertriples crash risk, and using alcohol increases this risk dramatically. In 2009,one in three fatally injured drivers tested positive for one or more drugs. It is no surprise that individual studies of injured drivers have found similar results. One such study of seriously injured drivers admitted to a shock-trauma center found 30% tested positive for alcohol but 50% tested positive for drugs. More than 26% of all seriously injured drivers tested positive for marijuana.

An expanding body of literature has documented the seriously impairing effects of marijuana on cognitive function andpsychomotor skills. Smoking marijuanadoublestheriskof crash. Recent research has demonstrated that chronic daily marijuana smokers display impairmenteven after three weeks of abstinence. This finding is particularly concerning given the legalization of marijuana for adults in Colorado and Washington. Since the commercialization of marijuana in Colorado in mid-2009, the prevalence of marijuana among fatally injured drivers has increased.

To address marijuana-impaired driving, these states have specific blood limits of 5ng/mL tetrahydrocannabinol (THC) for drivers age 21 and older. But in fact there is no blood impairment threshold for marijuana or any other drug because, unlike alcohol, consistent relationships between drug blood concentrations and impairmentdo not exist. The implications of these laws are vast. Data released from the Washington State Toxicology Laboratory showed that, for the first 6 months of 2013,43%of drivers arrested for impaired driving who tested positive for marijuana were below 5ng/mL THC. Similarly, in 2012, 72%of Colorado drivers arrested for impaired driving that were positive for marijuana tested below 5ng/mL THC. These drivers arrested for impairment who test positive for marijuana will likely not be prosecuted, putting others on the road at risk.

Reducing drugged driving recently has been identified as a national priority by the federal government. There is no simple solution. Law enforcement efforts are focused on better identification of drugs among individuals arrested for impaired driving. States are encouraged to pass zero tolerance laws where the presence of a drug not prescribed to the driver is a violation. As with drunk driving, it will take extensive public education and far more effective enforcement to turn back the tide of drugged driving.

Drugged driving is a global problem; other countries, including inAustralia andNew Zealand, have passed drug testing laws and implemented media campaigns that focus on marijuana impaired driving. In the United States, the changing legal status of marijuana and the likely resulting increased prevalence of marijuana impaired drivers on the roads pose a significant threat to public safety. For individuals using potentially impairing drugs as directed by their prescriber, it means understanding how a drug affects you before driving and using caution. We encourage patients to discuss this with their physician.

Analogous to the familiar admonition “Don’t Drink and Drive,” we need the clear message “Don’t Use Drugs and Drive.” Reducing drugged driving will save lives.

A version of this article appeared on the AAPS Blog on January 28, 2014.

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