Critics state that marijuana has been shown to damage brain cells and that this damage, in turn, causes memory loss, cognitive impairment, and difficulties in learning.
The original basis of this claim was a report that, upon postmortem examinations, structural changes in several brain regions were found in two rhesus monkeys exposed to THC. 51 Because these changes primarily involved the hippocampus, a cortical brain region known to play an important role in learning and memory, this finding suggested possible negative consequences for human marijuana users.
Additional studies, employing rodents, reported similar brain changes. However, to achieve these results, massive doses of THC—up to 200 times the psychoactive dose in humans—had to be given. In fact, studies employing 100 times the human dose have failed to reveal any damage.
In the most recently published study, rhesus monkeys were exposed through face-mask inhalation to the smoke equivalent of four to five joints per day for one year. When sacrificed seven months later, there was no observed alteration of hippocampal architecture, cell size, cell number, or synaptic configuration. The authors conclude:
“While behavioral and neuroendocrinal effects are observed during marijuana smoke exposure in the monkey, residual neuropathological and neurochemical effects of marijuana exposure were not observed seven months after the year-long marijuana smoke regimen.”
Thus, 20 years after the first report of brain damage in two marijuana-exposed monkeys, the claim of damage to brain cells has been effectively disproven.
No postmortem examinations of the brains of human marijuana users have ever been conducted. However, numerous studies have explored marijuana effect on brain-related cognitive functions. Many employ an experimental design—in which subjects are given marijuana in a laboratory setting, and then compared to controls on a variety of measures involving attention, learning and memory.
In a number of studies, no significant differences were detected. 54 In fact, there is substantial research demonstrating that that marijuana intoxication does not impair the retrieval of information learned previously. 55 However, there is evidence that marijuana, particularly in high doses, may interfere with users’ ability to transfer new information into longterm memory.
While there is general agreement that, while under the influence of marijuana, learning is less efficient, 57 there is no evidence that marijuana users—even longterm users—suffer permanent impairment. Indeed, numerous studies comparing chronic marijuana users with non-user controls have found no significant differences in learning, memory recall or other cognitive functions.