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Study: Higher THC/Blood Levels Not Positively Associated With Greater Fatal Accident Risk

Thursday, 03 April 2014

Study: Higher THC/Blood Levels Not Positively Associated With Greater Fatal Accident Risk

Porirua, New Zealand: Marijuana-positive drivers possess a slightly elevated risk of accident compared to drug and alcohol free divers, but this risk is not positively correlated with higher blood/THC levels, according to the results of a crash culpability analysis published online in the journal Accident Analysis and Prevention.

New Zealand researchers assessed the risk of accident associated with drivers who tested positive for the presence of drugs or alcohol in their blood compared to drug and alcohol free drivers in a cohort of 1,046 fatal vehicle crashes.

Alcohol-positive drivers were most likely to be culpable in fatal accidents (OR=13.7). Drivers who tested positive for drugs other than alcohol, including opiates, sedatives, cannabis, and stimulants, possessed an overall odds ratio of 3.5.

Drivers who tested positive for both cannabis and alcohol in their blood possessed were strongly associated with accident culpability (OR=6.9). By contrast, drivers who tested positive for the presence of marijuana only, as indicated by the presence of THC in blood, were weakly associated with accident culpability (OR=1.3). Notably, drivers who tested positive for the presence of THC in blood at levels of 5ng/ml or higher possessed no elevated rate of accident culpability (OR=1), but those with trace levels of THC (less than 2 ng/ml) did possess greater rates of culpability (OR=3.1).

According to a factsheet published by the US National Highway Traffic Safety Administration: "It is difficult to establish a relationship between a person's THC blood or plasma concentration and performance impairing effects. It is inadvisable to try and predict effects based on blood THC concentrations alone, and currently impossible to predict specific effects based on THC-COOH (metabolite) concentrations."

A road-performance study commissioned by the United States Department of Transportation similarly acknowledges: "One of the program's objectives was to determine whether it is possible to predict driving impairment by plasma concentrations of THC ... 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, or even some improvement."

Two states, Washington and Montana, define drivers who operate a motor vehicle with 5ng/ml or more of THC in blood as per se impaired. Colorado law presumes impairment in motorists who possess more than 5ng/ml of THC in their blood. Three states - Nevada (2ng/ml), Ohio (2ng/ml), and Pennsylvania (1ng/ml) - impose lower per se limits for THC in blood. Eleven states impose zero tolerance per se laws for the presence of cannabis. Other states require prosecutors to provide evidence of recent drug ingestion as well as evidence that a driver was under the influence of the substances that he or she had consumed in order to gain a DUI drug conviction.

In a 2013 review published in the Humboldt Journal of Social Relations, NORML Deputy Director Paul Armentano opined against the imposition of per se limits for cannabinoids, arguing: "The sole presence of THC and/or its metabolites in blood, particularly at low levels, is an inconsistent and largely inappropriate indicator of psychomotor impairment in cannabis consuming subjects. ... As additional states consider amending their cannabis consumption laws, lawmakers would be advised to consider alternative legislative approaches to address concerns over DUI cannabis behavior that do not rely solely on the presence of THC or its metabolites in blood or urine as determinants of guilt in a court of law. Otherwise, the imposition of traffic safety laws may inadvertently become a criminal mechanism for law enforcement and prosecutors to punish those who have engaged in legally protected behavior and who have not posed any actionable traffic safety threat."

According to a 2013 meta-analysis of 66 studies published in the journal Accident Analysis and Prevention, drug positive drivers for amphetamines (OR=6.19), opiates (OR=1.91) and benzodiazepenes (OR=1.17) possess the highest adjusted odds ratios of traffic accident injury, while drug positive drivers for penicillin (OR=1.12), antihistamines (OR= 1.12), cannabis (OR=1.10), and analgesics (OR=1.02) possess the lowest odds ratios.

For more information, please contact Paul Armentano, NORML Deputy Director, at: paul@norml.org. Full text of the study, "The culpability of drivers killed in New Zealand road crashes and their use of alcohol and other drugs," is available from Accident Analysis and Prevention."