While, there have been many studies on the effects of cannabis on memory, more investigation remains before conclusive results can be made. This is due both to the nature of human memory, which is not a simple measurable idea and to the make up of the cannabis plant itself which is comprised of more than 600 ingredients and 60 different cannabinoids in addition to THC, some of which have counteractive effects.
There is widespread agreement both scientifically and practically a single heavy dose of THC (delta-9-tetrahydrocannabinol) has an acute impairment of on verbal and working memory. The effects are less acute for regular heavy users of cannabis, defined in one study as those who smoked more than 94 joints a week. Studies have shown that there no persistent impairment of memory functions within 24 to 48 hours of a single dose of THC.
The questions arise concerning the long term effect on memory by heavy use of cannabis. These studies have been inconclusive for heavy (or chronic) adult users of cannabis, which is largely due to the difficulty in creating a baseline for test subjects and tracking the dosage of THC and other cannabinoids in cannabis over the long term. Brain scans do show some morphological differences between heavy users and non users of THC in areas of the brain generally associated with memory however much work needs to be done here before conclusive results can be stated.
One area for which there is a growing body of conclusive research is the adverse cognitive and morphological effects of heavy long term use of high doses of THC by users under the age of 18 while the brain is still developing.
A final area of interest is the positive effects of cannabinoids on memory functions. Studies have shown that CBD have a counteractive effect on the acute memory impairments caused by THC. This is a growing area of study regarding the effects of cannabis use on memory.