Effects of Cannabis on Memory
& Cognitive Function
of cannabis on tests of short-term memory are well-established.
Miller reported "The single, most
consistently reported behavioural effect of cannabinoids
in humans is an alteration of memory functioning."
studies of the effect of cannabis on memory have used
digit span or verbal recall tests where dose-related
decrements in performance have been demonstrated,
including errors of omission (correct responses missing)
or intrusion (incorrect responses made). However,
these appear to represent errors in detail rather
than the main factual elements in narrative, and the
recall of previously-learned material does not appear
to be affected.
et al found no effect of cannabis on recall of "negative"
emotionally charged words compared to other words,
although "positive" words were recalled
more frequently than expected. In a thorough review
of the cognitive effects of cannabis, Solowij concluded
that cannabis may affect the acquisition of information
due to interruption of attention and concentration,
and may interfere with transfer from short-term to
dispute as to the duration of any cannabis effects
on memory and performance. In a study of 3 regular
marijuana smokers, Heishman et al found no subjective
intoxication or tachycardia 24 hours after marijuana
administration, although there remained impairment
on arithmetic and recall tasks, concluding "These
preliminary results suggest that marijuana can adversely
affect complex human performance up to 24 hours after
smoking.". A 1985 study by Chait et al
had suggested the existence of a "hangover"
syndrome after "significant changes
were observed on two subjective effects scales and
on the time production task after active, but not
placebo, marijuana" roughly 9 hours after
smoking, although other measures were unaffected.
However, in a later study of 12 regular users by Chait,
testing subjective ratings and a battery of psychomotor
and cognitive tests the morning after use, found few
statistically significant differences, these being
of negligible magnitude including counterintuitive
results, and conceded "marijuana
smoking was not associated with a "hangover" syndrome
similar to those reported after use of alcohol or
In a separate
study, Heishman et al reported "Significant
memory impairment was observed on a forward and reverse
digit span task, and performance was impaired on the
digit symbol substitution task by the high, but not
low, dose of marijuana. Performance on a divided attention
task was not affected by marijuana.",
also finding that users would titrate the quantity
of smoke of lower and higher strength marijuana by
taking smaller puffs, inhaling less and for shorter
duration with the higher potency material.
al, studying 26 Indian cannabis users, reported "Compared
with a control group, the cannabis users were found
to react slowly in perceptuo-motor tasks, but not
to differ in intelligence or memory tests. The users
suffered disability in personal, social and vocational
areas and indicated higher psychoticism and neuroticism
scores." The same team earlier reported
a study of regular users averaging 150mg THC per day,
finding "the cannabis users were
found to react more slowly, to be poorer in concentration
and time estimation, to have higher neuroticism and
greater perceptuo-motor disturbance".
In a study
of cognitive function among 12 male volunteers, Hooker
& Jones reported "A significantly
greater number of short story omissions and intrusions
occurred in delayed free recall after marijuana. Immediate
and sustained attention, controlled retrieval from
semantic memory, and speed of reading and naming colors
were not affected." Miller et al reported
"Free recall was reduced in a dose related manner by the drug, but recognition
memory was unaffected.".
et al found oral marijuana (0.3mg/kg THC) to have
no effect on fluency, flexibility, elaboration, uniqueness,
associational thinking or creativity. Individual differences
in ability are clearly of major importance in determining
performance, Miller et al reported "Recall performance following marijuana intoxication was positively related
to level of recall performance in the placebo condition.".
In a separate study, the same team reported "marijuana
significantly decreased immediate and final free recall
but only slightly influenced recognition memory."
required volunteers to consume 20mg THC in a short
"fast" period (10 min) or long "slow"
period (45min), and found "In fast
conditions, THC induced greater difficulties with
tracking information over time, greater disruptions
of self-other interpersonal perceptions, and more
persecutory ideation that did alcohol or placebo.
Similar but less marked differences were found in
the slow conditions. ...changes in tracking difficulties,
self-other metaperspectives, and persecutory ideation
were substantially and significantly correlated."
In a study
of the effects on long term memory, Darley et al reported
"Marijuana had no effect on recall
or recognition performance. These results do not support
the view that marijuana provides access to facts in
long-term storage which are inaccessible during non-intoxication.
" The presence of retrieval cues does not
appear to affect recall of word lists
have consistently demonstrated significant impairment
of short-term memory for number sequences or verbal
recall, although not for major events or factual elements
of narratives. The effects of cannabis are usually
subtle, and although statistically significant, the
magnitude of any impairment tends to be dose-related
and subject to tolerance of the drug"s effects,
on the personal characteristics of the user, and on
the circumstances in which the drug is taken.
may increase the likelihood of minor errors of time
perception, verbal recall and similar factors, but
not affect recall of important actions observed or
experiences undertaken. In a regular user of cannabis,
acute intoxication by the drug would have little effect
appear to disrupt short-term memory by interfering
with the filtering of information, such that a greater
volume of information reaches consciousness, overwhelming
the ability to store, or prioritise such information
for storage in memory.