- 36 -
adjustment for all confounders that remain stable over time,” such as personality traits, driving
ability, physical limitations, or the presence of other drugs. Thus, this design represented an
improvement over earlier observational studies that were only partly successful in controlling
for confounders. The 68 subjects who reported sustaining injuries after driving a motor vehicle
became the core subjects for this study. The results showed a relative-risk (RR) of 5.8 during the
first hour after cannabis use among subjects who had ingested cannabis but were free of other
drugs or alcohol (compared to the control period for the same drivers, in which they were free
of all psychoactive substances), which rose to 10.9 for subjects who had ingested alcohol as well
as cannabis. The RR for both groups dropped to 2.2 and 1.9, respectively, during the second
hour of this study. Thus, combining alcohol with cannabis resulted in an 88 percent increase in
the risk of sustaining an injury while driving.
Ramaekers JG, Theunissen EL, De Brouwer M, Toennes SW, Moeller MR, & Kauert G (2011). Tolerance
and cross-tolerance to neurocognitive effects of THC and alcohol in heavy cannabis users.
Psychopharmacology. 214:391-401.
This double-blind, placebo-controlled study investigated the hypothesis, based largely on
numerous animal studies, that heavy cannabis users develop cross-tolerance to the impairing
effects of alcohol. The authors administered critical tracking (CT), divided attention (DA), stop-
signal (SS), and Tower of London (CF) tests to 21 daily cannabis users over a seven-hour period,
during which the subjects drank alcohol in placebo, low-level, and high-level concentrations. The
authors hypothesized, based on animal testing data, that cannabis users who are highly tolerant
to the effects of THC may also be cross-tolerant to the effects of alcohol. However, while the
study found that THC generally did not affect neurocognitive performance in heavy users,
alcohol strongly affected it. Moreover, combining alcohol with THC resulted in diminished
performance on DA test compared to alcohol alone conditions.
Ramaekers JG, Kauert G, van Ruitenbeek P, Theunissen EL, Toennes SW, & Moeller MR (2009).
Neurocognitive performance during acute THC intoxication in heavy and occasional cannabis users.
Journal of Psychopharmacology.23:266-277.
This study used a double-blind, placebo-controlled, mixed-model design to quantify the
relationship between tolerance (in experienced users) to THC and the apparent reduction in the
observed degree of performance deficit. It also confirmed that THC alone generated
performance deficits for occasional users in critical tracking, divided attention capability,
reaction time, and cognitive function. By contrast, neither CT nor DA capabilities were
compromised by the same doses in heavy users.
Ramaekers JG, Berghaus G, van Laar M, & Drummer OH (2004). Dose related risk of motor vehicle
crashes after cannabis use. Drug and Alcohol Dependence.73:109-119.
This study reviewed earlier studies and found that all those relying on carboxy-THC as a measure
of driver impairment (e.g., based on urine-testing) found odds-ratios of about 1.0 and below,
thereby driving the erroneous conclusion that cannabis is not impairing.