Marijuana and the Cannabinoids

Medical and Health Consequences of Marijuana 243
cerebellum with increased prenatal exposure to marijuana when challenging the
response inhibition neural circuitry. Prenatally exposed offspring had significantly
more commission errors than nonexposed participants, but all participants were able
to perform the task with more than 85% accuracy. These findings suggest that prenatal
marijuana exposure is related to changes in neural activity during response inhibition
that may last into young adulthood (51).
7. EFFECTS ON THE BRAIN:
COGNITIVE, PSYCHOLOGICAL, AND MENTAL CONSEQUENCES
Research by Pope and Yurgelum-Todd (52), Kouri et al. (53), Solowij et al. (54),
and Block and Ghoneim (55) has shown that chronic use of marijuana was associated
with impairment of cognition, particularly affecting short-term memory and executive
functioning in humans; and this impairment did not recover after abstaining from heavy
use of marijuana (up to 5000 times in a lifetime) for at least 24 hours (52), 7 days (56),
or 6 weeks (54). However, in the study of Pope and colleagues (57), the subjects did
recover after 28 days of abstinence from marijuana use. In studies by Pope and colleagues
(52,56,57), the subjects smoked marijuana up to 5000 times in their lifetime
(8-15 years), whereas in the study by Solowij et al. (54), the subject had smoked approx
6 g of marijuana each day for about 17 years. Many other older studies have also
reported that marijuana use is associated with impairment of short-term memory and
not “old” memory.
Pope and Yurgelum-Todd (52) found that heavy use of marijuana is associated
with cognitive impairment in college undergraduate students. The researchers enrolled
two groups of students—65 “heavy users” (38 male, 27 female), who had smoked
marijuana a median of 29 days in the past 30 days (range 22–30) and who also displayed
cannabinoids in their urine, and 64 “light users” (31 male, 33 female), who had
smoked a median of 1 day in the previous 30 days (range 0–9) and who displayed no
urinary cannabinoids. All of the subjects were assessed by several neuropsychological
tests when they were abstinent from marijuana and other drug use for at least 19 hours.
The outcome measures were general intellectual functioning, abstraction ability, sustained
attention, verbal fluency, and ability to learn and recall new verbal and
visuospatial information. Heavy users displayed significantly greater impairment than
light users in attention/executive functions, as evidenced by greater perseverations on
card sorting and reduced learning of word lists. These differences remained after controlling
for potential confounding variables, such as estimated levels of premorbid
cognitive functioning, and for use of alcohol and other substances in the two groups. It
is not clear whether this cognitive impairment is a reslut of a residue of drug in the
brain, a withdrawal effect from the drug, or a frank neurotoxic effect of the drug.
Similarly, Fletcher and colleagues (58) reported cognitive impairment from
chronic marijuana use, but in older subjects. They studied two cohorts of older chronic
cannabis-using and cannabis-nonusing adult men. Both cohorts were comparable in
age and socioeconomic status. Polydrug users and users who tested positive for use of
cannabis at the time of cognitive assessment after a 72-hour abstention period were
excluded. The older cohort (17 users, 30 nonusers; mean age 45 years) had consumed