Such mild cognitive deficits may not endanger anyone if a marijuana user lazes on the couch, but it is a different story when someone takes that high on the road. In driving-simulation and closed-course studies, people on marijuana are slower to hit the brakes and worse at safely changing lanes. Investigators still debate, however, at what point these impairments translate to more traffic accidents. A 2009 study found an increased risk of accidents for levels of THC higher than five nanograms per milliliter of blood, which some evidence indicates is as impairing as a blood alcohol concentration around the legal limit of 0.08 percent. Typically one would have to take several puffs of a joint to reach such a concentration. Consequently, voters in Washington State have adopted 5 ng/mL as the upper threshold for drivers.
Enforcing that limit presents a technical challenge, however. Unlike alcohol, marijuana cannot be detected with a relatively unobtrusive Breathalyzer test. Police officers would have to look for it in blood—something that often requires a warrant. “There is currently no practical method for law-enforcement officers at the scene to collect blood samples from suspected DUI cannabis drivers in a timely manner,” says Paul Armentano, deputy director of the Washington, D.C.–based National Organization for the Reform of Marijuana Laws, which advocates the legalization of marijuana. Instead of using a blood test, Armentano says that police should look for poor maneuvering and the smell of pot wafting from the vehicle.
Although marijuana's immediate effects are relatively easy to monitor in the lab, the drug's long-term effects on body and mind are harder to determine. So far the results—which admittedly are subject to multiple interpretations—indicate the need for caution. In one recent study, clinical psychologist Madeline Meier of Duke University and her colleagues examined data from 1,037 New Zealanders. They found that people who began using pot earlier in life and used it most frequently over the years experienced an average decline of eight IQ points by the time they turned 38. By comparison, those who never smoked pot had an average increase of one IQ point by the same age.
A reanalysis of the New Zealand data by Ole Røgeberg of the Ragnar Frisch Center for Economic Research in Oslo, however, suggested that the IQ difference could be explained by socioeconomic factors. People who start smoking marijuana at an earlier age are often less intelligent to begin with. Even if this is true, Meier says, her study shows that the IQ drop is greatest for those who started smoking pot as teenagers rather than in adulthood, indicating a worrisome cumulative effect regardless of intelligence. This finding, she thinks, makes it all the more important to discourage the early use of marijuana among teens.
Increasingly potent marijuana of recent years may be driving a sharp rise in cannabis addiction among adolescents, according to a report released last year by the American Society of Addiction Medicine. Between 1993 and 2008, the average concentration of THC in confiscated marijuana jumped from 3.4 to 8.8 percent. Meanwhile hospital and rehabilitation center admission rates for minors abusing marijuana soared by 188 percent between 1992 and 2006. In contrast, admissions for alcohol abuse for the same group over the same period declined by 64 percent.
In addition to tracking levels of THC itself, some researchers have focused on the dangers of lingering contaminants in marijuana sold on the street. Dealers typically sell cannabis by weight, so some use sand or glass beads to make their products heavier. Breathing in these particles over the years may inflame and eventually scar the lungs. An analysis published last year of data on more than 5,000 Americans did not find a decline in lung function among individuals who smoked joints two or three times a month over two decades. The authors emphasize, however, that they did not assess the effect of daily use on lung health. “Somebody should do that study if marijuana is going to become legalized and prescribed” more widely, says Mark Pletcher, an epidemiologist at the University of California, San Francisco, who co-wrote the paper.