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Our feature story this week examines the unusual controversy surrounding I-502, the ballot initiative that proposes legalizing, selling and taxing marijuana in Washington.

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The Science Behind Stoned Driving and THC Blood Levels

medical-marijuana-brain 150x120.jpg
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Our feature story this week examines the unusual controversy surrounding I-502, the ballot initiative that proposes legalizing, selling and taxing marijuana in Washington. The measure would also establish a limit for THC blood content for drivers -- sort of like the .08 limit for booze -- and that has caused some longtime crusaders for marijuana legalization to vehemently oppose it. They worry the proposed standard is low enough that stoned drivers would be at risk of being busted for DUI, even if they're driving stone cold sober. But is that fear supported by science?

The answer isn't necessarily a simple yes or no, but there are multiple studies that suggest the heaviest pot users could test positive for active THC in their blood up to two days after their last toke. Those findings are especially concerning for medical patients, who aren't exempt from the proposed DUI law in I-502.

The initiative calls for a limit of 5 nanograms of active THC per milliliter of whole blood for drivers 21 and older, and a zero tolerance standard for younger motorists. What doesn't count for much in court is carboxy-THC, the inactive metabolite that can linger in the body for weeks after an individual's most recent bong hit.

That 5 nanogram standard is largely based on a 2005 study from a team of international scientists led by Germany's Dr. Franjo Grotenherman. Here's more on that from our feature:

The researchers caution that "empirical evidence on cannabis and driving is still much less conclusive than that for alcohol," but suggest a five nanogram limit, "may achieve a reasonable separation of unimpaired from impaired drivers, who pose a higher risk of causing accidents."

It's important to note, though, that Grotenhermen and his colleagues devised their standard for infrequent users. The researchers caution that routine tokers "may present measurable THC concentrations at all times." They also point out that exposure to secondhand smoke could result in false positives, meaning a zero-tolerance standard like the one proposed for minors in I-502, "facilitates law enforcement but is not based on science and does not target impaired drivers."

The study (click here for PDF) is nearly 50 pages long, however, and those brief quotes only scratch the surface of the findings. We'll get to a few more detailed excerpts momentarily, but first some background info on THC blood levels might come in handy.

DUI prosecutions typically focus on the level of active THC in the blood, a figure that varies widely based on the potency of the pot, the frequency of use, and the manner in which it was taken into the body. In smoking, the active THC level generally spikes at 50-plus nanograms immediately after inhalation, then dissipates rapidly, usually disappearing within one to four hours for occasional users. The difference for hardcore stoners, or patients who medicate almost continuously around the clock, is that they already have background levels of active THC leftover from their last high.

Grotenhermen and his colleagues parsed more than 140 epidemiological and experiential studies, summarizing many of the finding in their report. Here's what they say about active THC, driving, and frequent users:

"Frequent users of cannabis may show THC concentrations in whole blood of more than 2 ng/mL for up to 48 hours after the last use."
"the blood serum of moderate or heavy users of cannabis may contain more than 2 ng/ml of THC at 24-48 hours after smoking a single joint because of the storage in tissue and slow release of the fat-soluble THC."
"Even frequent users of cannabis do not seem to have a higher accident risk than nonusers, as long as they are not under the acute influence of the drug, i.e. there appear to be no extended effects of cannabis use on traffic safety beyond the period of acute impairment"
The "acute period of impairment" is the first one to two-and-a-half hours after smoking. Grotenhermen also writes, however, that THC blood levels do not necessarily correlate to being high as a kite. "Maximum THC blood concentrations have already dropped significantly before maximum psychotropic effects occur," the study says. "Consequently, during the first hour after taking cannabis, during which THC is distributed in the body, there is no unambiguous relationship between subjective high and blood THC concentration."

Another interesting paper on the subject comes from NIDA, the National Institute on Drug Abuse (click here to read in full.) They recruited 25 of the "heaviest, longest-term cannabis smokers" they could find, then monitored their THC blood levels for a week, keeping them under 24-hour observation to ensure they weren't secretly sparking joints and skewing the data. The NIDA researchers found that, "substantial whole blood THC concentrations persist multiple days after drug discontinuation in heavy chronic cannabis users."

Here's a particularly relevant excerpt:

"Surprisingly, on the seventh day of monitored cannabis abstinence, six full days after entering the unit, six participants' whole blood specimens contained THC greater than 0.25 nanograms per milliliter, with three at greater than one nanograms per milliliter."
While those levels are well below I-502's proposed 5 nanogram limit for drivers 21 and up, it's bad news the 16-21 group, which will be subject to a zero tolerance standard should the initiative become law. NIDA's Dr. Erin Karschner even addresses this issue in her paper, though she's obviously not speaking specifically about I-502 when she writes, "These findings also may impact on the implementation of per se limits in driving under the influence of drugs legislation."

Another study that weighs-in on the issue of frequent pot users and lingering active THC is from 2008, from the Institute of Forensic Toxicology, and the Institute for Legal Medicine at Goethe-University in Frankfurt, Germany:

"It must be cautioned that cannabinoid blood concentrations from heavy users in a late elimination phase may be difficult to distinguish from concentrations measured in occasional users after acute cannabis use."
It's important to note that decades of research have also shown that smoking weed almost certainly affects driving performance, at least for the first hour or so after lighting up.

Citing the large body of research on the subject, Grotenhermen and his collaborators write that THC affects "highly automated driving functions" such as "tracking performance," and that "drivers under the influence of cannabis generally show more awareness of their impairment than drivers on alcohol and are able to make the correct response if given a warning. However, where events are unexpected such compensation is not always possible."

The international scientists say a blood alcohol content of .08 percent corresponds approximately to a THC concentration in whole blood of 4-5 nanograms per milliliter (again, for infrequent users), and they suggest waiting about three hours after smoking a "strong social dose" to get below that limit.

There are, however, studies that make it seem as though driving while high might not be so dangerous after all, especially after that initial buzz has worn off. This is from Dr. Jan Ramaekers, Head of the Department of Neuropsychology and Psychopharmacology at Maastricht University in The Netherlands:

"THC generally did not affect task performance. However, combined effects of THC and alcohol on divided attention were bigger than those by alcohol alone...heavy cannabis users develop tolerance to the impairing effects of THC on neurocognitive task performance."
And this comes from R. Andrew Sewell, from VA Connecticut Healthcare and the Yale University School of Medicine:
"Cannabis and alcohol acutely impair several driving-related skills in a dose-related fashion, but the effects of cannabis vary more between individuals than they do with alcohol because of tolerance, differences in smoking technique, and different absorptions of [THC]... Because of both this and an increased awareness that they are impaired, marijuana smokers tend to compensate effectively while driving by utilizing a variety of behavioral strategies. Combining marijuana with alcohol eliminates the ability to use such strategies effectively, however, and results in impairment even at doses which would be insignificant were they of either drug alone."
And, finally, one study that was referenced in our feature was commissioned by the U.S. Department of Transportation in 1993. They observed stoned drivers in action, and crunched the numbers from traffic fatalities. Here's what they concluded:"
In high-density urban traffic...THC did not impair driving performance yet the subjects thought it had." Marijuana And Actual Driving Performance." Also, review of the role of drug use in fatal accidents reported, "[T]here is little if any evidence to indicate that drivers who have used marijuana alone are any more likely to cause serious accidents than drug free drivers...

...One of the program's objectives was to determine whether it is possible to predict driving impairment by plasma concentrations of THC and/or its metabolite, THC-COOH, in single samples. The answer is very clear: it is not. Plasma of drivers showing substantial impairment in these studies contained both high and low THC concentrations; and drivers with high plasma concentrations showed substantial, but also no impairment, and even some improvement."

So there you have it: according to the U.S. Government, smoking weed actually makes some people better drivers. But does that study, and all the aforementioned science put together amount to a good enough reason to oppose a measure that would make Washington the first state to legalize marijuana? That's a thorny question that voters will have to decide for themselves this November.

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