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Research shows that smoking pot affects drivers weeks after last use

The study is the first of its kind to study the effects of marijuana on driving up to one month after the smoker's last use of the drug.


With recreational marijuana use now legal in two states (Colorado and Washington), the researchers are increasingly studying the potential affects of the drug. With that in mind, researchers at the Chemistry and Drug Metabolism, Intramural Research Program have published a study examining how long pot smokers remain “impaired” after using marijuana.

So what did researchers find? According to the study’s conclusion, “Our results demonstrate, for the first time as far as we are aware, Cannabinoids can be detected in blood of chronic daily cannabis smokers during a month of sustained abstinence. This is consistent with the time course of persisting neurocognitive impairment reported in recent studies.”

The findings mark the first time a test has shown that cannabis can be detected in the blood of chronic daily marijuana smokers for a month after their last use.

As for why this type of study has never been conducted, Dr. Marylin Huestis of the National Institutes of Health and author on the paper, said: “These data have never been obtained previously due to the cost and difficulty of studying chronic daily cannabis smoking over an extended period.”

The study, which was published in Clinical Chemistry, the journal of the American Association of Clinical Chemistry (AACC), looked at 30 male participants over the course of 33 days.

While researchers established how long cannabinoids remain in the blood, they did not examine exactly what “impaired” driving entails. That being said, a February study in the British Medical Journal found that marijuana nearly doubles the risk of a vehicle collision. Examples of impaired marijuana driving include following cars too closely and swerving in and out of lanes.

Furthermore, among impaired drivers, fatally injured drivers and crash victims, the most prevalent illegal drug that has been detected is marijuana, according to the U.S. National Institute on Drug Abuse.

Conversely, a recent study by CNN found that pot smokers only became impaired when researchers got them well above the legal THC limit, which is 5 nanograms in Colorado and Washington. Click HERE to watch a video of the experiment.

Although recreational marijuana use is legal in just two states, the Marijuana Policy Project is currently pursuing marijuana-policy-reform in Alaska, California, Maine, Massachusetts, Nevada, Oregon and Rhode Island.

Additionally, 18 states and Washington D.C. have legalized the use of medical marijuana.


Krymsun (not verified)    March 5, 2013 - 12:42PM

Is Driving High on Marijuana Safer Than Driving Drunk? [ or driving sober?!! ]
Studies have shown marijuana users are Safer Drivers than either drunk drivers, or sober ones.

For decades, marijuana advocates have argued that pot has a significantly different effect on driving ability than alcohol. But if you take the word of one auto insurance company, stoned is actually the safest way to drive. is making that case based on years’ worth of scientific studies, including some from the US National Highway Transportation Safety Administration that found motorists under the influence of marijuana tended to drive slower and have accident responsibility rates lower than those of drug-free drivers.

Krymsun (not verified)    March 5, 2013 - 12:44PM

Ethanol, marijuana, and other drug use in 600 drivers killed in single-vehicle crashes in North Carolina, 1978-1981.

[ Journal of Forensic Sciences, Oct., 1984 by A. P. Mason, A. J. McBay ]

Blood ethanol concentrations (BECs) were usually high; 85.5% of the drivers whose bloods contained ethanol and 67.8% of all drivers had BECs greater than or equal to 1.0 g/L. Drug concentrations were usually within or were below accepted therapeutic or active ranges. Only a small number of drivers could have been impaired by drugs, and most of them had high BECs. Multiple drug use (discounting ethanol) was comparatively rare. Ethanol was the only drug tested for that appears to have a significantly adverse effect on driving safety.

Krymsun (not verified)    March 5, 2013 - 12:45PM

Cannabis use is associated with only marginal increases in traffic accident risk, comparable to anti-histamines and penicillin.

An investigator from Aalborg University and the Institute of Transport Economics in Oslo assessed the risk of road accident associated with drivers’ use of licit and illicit drugs, including amphetamines, analgesics, anti-asthmatics, anti-depressives, anti-histamines, benzodiazepines, cannabis, cocaine, opiates, penicillin and zopiclone (a sleeping pill).

His study reviewed data from 66 separate studies evaluating the use of illicit or prescribed drugs on accident risk; the study found that cannabis was associated with minor, but not significantly increased odds of traffic injury (1.06) or fatal accident (1.25). By comparison, opiates (1.44), benzodiazepine tranquillizers (2.30), anti-depressants (1.32), cocaine (2.96), amphetamines (4.46), and the sleeping aid zopiclone (2.60) were all associated with a greater risk of fatal accident than cannabis. Anti-histamines (1.12) and penicillin (1.12) were associated with comparable odds to cannabis.

Krymsun (not verified)    March 5, 2013 - 12:47PM

One study, entitled "Medical Marijuana Laws, Traffic Fatalities, and Alcohol Consumption" conducted in November 2011 provides evidence that marijuana is a safer substitute for alcohol when it comes to health and also makes for safer drivers.

Top Ten Reasons Marijuana Users Are Safe Drivers

When you combine all of the main results of these two decades worth of scientific research studies, the following 10 reasons marijuana drivers are safer than drunk drivers comes out like this:

1. Drivers who had been using marijuana were found to drive slower, according to a 1983 study done by U.S. National Highway Transportation Safety Administration (NHTSA). This was seen as a factor in their favor, since drivers who drank alcohol usually drove faster and that is part of the reason they had accidents.

2. Marijuana users were able to drive straight and not have any trouble staying in their own lanes when driving on the highway, according to a NHTSA study done in 1993 in the Netherlands. The study determined also that the use of marijuana had very little effect on the person’s overall driving ability.

3. Drivers who had smoked marijuana were shown to be less likely to try to pass other cars and to drive at a consistent speed, according to a University of Adelaide study done in Australia. The study showed no danger unless the drivers had also been drinking alcohol.

4. Drivers high on marijuana were also shown to be less likely to drive in a reckless fashion, according to a study done in 2000 in the UK by the UK Transport Research Lab. The study was done using drivers on driving simulators over a period of a month and was actually undertaken to show that pot was a cause for impairment, but instead it showed the opposite and confirmed that these drivers were actually much safer than some of the other drivers on the road.

5. States that allow the legal use of marijuana for medical reasons are noticing less traffic fatalities; for instance, in Colorado and Montana there has been a nine percent drop in traffic fatalities and a five percent drop in beer sales. The conclusion was that using marijuana actually has helped save lives. Medical marijuana is allowed in 18 states in the U.S.

6. Low doses of marijuana in a person’s system was found by tests in Canada in 2002 to have little effect on a person’s ability to drive a car, and that these drivers were in much fewer car crashes than alcohol drinkers.

7. Most marijuana smokers have fewer crashes because they don’t even drive in the first place and just stay home thus concluded more than one of these tests on pot smoking and driving.

8. Marijuana smokers are thought to be more sober drivers. Traffic information from 13 states where medical marijuana is legal showed that these drivers were actually safer and more careful than many other drivers on the road. These studies were confirmed by the University of Colorado and the Montana State University when they compared a relationship between legal marijuana use and deaths in traffic accidents in those states. The studies done by a group called the Truth About Cars showed that traffic deaths fell nine percent in states with legal use of medical marijuana.

9. Multiple studies showed that marijuana smokers were less likely to be risk takers than those that use alcohol. The studies showed that the marijuana calmed them down and made them actually pay more attention to their abilities. All of these tests and research studies showed that while some people think that marijuana is a major cause of traffic problems, in reality it may make the users even safer when they get behind the wheel.

10. Marijuana smoking drivers were shown to drive at prescribed following distances, which made them less likely to cause or have crashes.

.. stick *that* in your pipe, and smoke it!