Many cannabis connoisseurs assert that their habit is less harmful than alcohol, despite evidence to the contrary. A new study by the National Institute on Drug Abuse, published on June 23, compared the effects of cannabis to alcohol in regard to impaired driving. It was a difficult task because the impact of smoking marijuana on driving performance, many variables affect impairment: the potency of the drug, the tolerance of the smoker, when the drug was taken, how the drug was taken.
For the study, the researchers used the most technologically advance driving simulator available to simulate of its kind to mimic real-life situations. They found that individuals driving with blood concentrations of 13.1 micrograms per liter (µg/L) tetrahydrocannabinol (THC), the main psychoactive ingredient in marijuana, exhibited increased weaving within the lane, similar to those with a 0.08 breath alcohol level, which is the legal threshold for impaired driving in many states. The investigators also assessed the combined effects of alcohol consumption and smoking marijuana. They found that the combination had an additive effect: drivers using both substances weaved within lanes even if their blood THC and alcohol concentrations were below the impairment thresholds for each substance alone. Moreover, alcohol, but not marijuana, increased the number of times the car actually left the lane and the speed of weaving.
For the study, more than 50% of the participants controlled their marijuana inhalations (a technique known as titration) so they had consistent blood THC peak concentrations, regardless of the percentage of THC in the marijuana (the marijuana ranged from 2.9% to 6.7% THC). The authors note that this standardization process they used reveals that past driving studies based on cannabis dose rather than blood THC may have overlooked the importance of dose titration. Furthermore, the investigators found that small amounts of alcohol significantly increased peak THC concentrations.
The authors explain that THC concentrations decrease rapidly during the time required to collect a blood specimen in the United States, generally within two to four hours. Oral fluid (saliva) tests for THC can be conducted roadside without this long wait; however, oral fluid THC was found to have a two to five fold greater variability than blood. This points out that oral fluid may be an effective screening tool for detecting recent marijuana use by a driver; however, it may not be a precise measure of the level of impairment.
Additional information regarding drugged driving is available at this link.