As a growing number of states move to legalize marijuana, a consensus has yet to be reached on an important question: How does weed affect someone’s driving ability? And how can lawmakers ensure the roads are as safe as possible if these laws are passed? A new study may help find the answers.
According to the journal of the American Association of Clinical Chemistry, pot can be detected in the blood of daily smokers up to a month after the last puff is taken. The implication: Marijuana may have a longer-lasting effect on someone’s motor skills than previously thought.
Marilyn Huestis, a senior investigator at the National Institute on Drug Abuse and author of the research paper, says this type of data hadn’t been collected before. Studying daily pot smokers over a long period of time can be costly, which makes these findings unprecedented.
“These data add critical information to the debate about the toxicity of chronic daily cannabis smoking,” Huestis said in a press release. (Huestis couldn’t be reached for comment).
To conduct the study, 30 chronic marijuana smokers lived in a research center for 33 days; they quit smoking marijuana and had blood samples drawn daily. Twenty-seven of the 30 participants tested positive for THC (marijuana’s psychoactive chemical) when they were admitted. After 30 days, 23 participants still had detectable amounts of THC in their blood.
According to the researchers, the test subjects still had enough THC in their systems to harm neurocognitive ability, which can include concentrating for long periods of time or multitasking. In other words, they may have not been fit to drive a car.
Heustis and her colleagues suggest these findings could provide a basis for new laws about driving under the influence of THC, but experts say many questions still remain.
The dangers of driving while under the influence of marijuana
Surprisingly, there’s little data available about how long-term marijuana use affects someone’s driving. In the study, Huestis and her colleagues write that the immediate effects of marijuana — usually two to four hours after smoking — are well documented. However, “the persistence of chronic impairment is less clear.”
They add that marijuana is second only to alcohol for causing impaired-driving crashes. But really, just how dangerous is it to be stoned behind the wheel?
“The answer to this question really has produced mixed results,” says Dr. David Sack, CEO of Promises Treatment Centers and an expert on substance abuse. “What we know is that marijuana affects reaction times, concentration, decision-making, alertness, balance and coordination, along with other indicators of driving performance, for about two-to-four hours after use. After that, it’s not really clear. We need more studies.”
The problem is that once THC is in the body, it doesn’t decrease at a predictable rate. Unlike alcohol, which begins to leave the system shortly after the last drink, THC has a tendency to hang around, apparently for weeks. But just because there’s detectable THC in the blood doesn’t necessarily mean that a person shouldn’t be operating a vehicle.
“There were a few things that really bothered me about this study, and one of them is the conclusion that people may be impaired by cannabis for 30 days,” says Paul Armentano, deputy director of the pro-pot National Organization for the Reform of Marijuana Laws (NORML). “Are they really trying to suggest that people are high for 30 days? There’s simply no evidence that points to this type of residual cognitive impairment.”
Drugged driving laws
Right now, 10 states have zero-tolerance policies toward driving under the influence of marijuana. In Arizona, Delaware, Georgia, Illinois, Indiana, Iowa, Michigan, Rhode Island, Utah and Wisconsin, drivers pulled over with any amount of THC in their systems can be ticketed.
Four other states — Nevada, Ohio, Pennsylvania and Washington — set limits for the amount of THC allowed in the blood while behind the wheel. (For a state-by-state list of drugged-driving laws and penalties, visit the NORML website.)
This is where the debate gets most heated. If THC is detectable in the blood of chronic users up to 30 days after the last toke, is it really fair for states to impose such strict drugged-driving laws?
“Unlike the approach we take with alcohol, it isn’t particularly useful to base drugged-driving laws on specific THC blood levels,” Sack says. “For one thing, alcohol’s effects are relatively consistent across the board. But people vary in their sensitivity to marijuana.”
Sack says that since marijuana stays in the body for long periods of time, habitual users who have built up a high tolerance — including users of medical marijuana — would test positive almost all of the time, even if their driving isn’t impaired.
Even the National Highway Traffic Safety Administration (NHTSA) advises against drugged-driving laws based on blood tests. On its “Drugs and Human Performance Fact Sheet,” NHTSA writes: “It is inadvisable to try and predict effects based on blood THC concentrations alone.”
What’s more, Armentano points to a 2012 study that shows states with the strictest drugged-driving laws have shown no decrease in traffic deaths related to marijuana.
“Our results clearly indicate that, as currently implemented, laws that make it illegal to drive with detectable levels of a controlled substance in the system have little to no effect on traffic fatalities,” says D. Mark Anderson, the study’s lead author and assistant professor of economics at Montana State University.
In the end, Armentano suggests that far more research and data collection needs to be done before policymakers start passing stricter laws about driving under the influence of marijuana.
“We can’t just jump to discussing new laws before we even understand impairment,” Armentano says. “It may be very convenient for states to say, ‘If it’s in your system, you’re guilty.’ But it’s just not that simple.”