Marijuana and Memory: Why You Can’t Remember Now but Why You Will Remember Later

“…What were we just talking about?”

You or someone you know may have uttered this phrase after one too many hits, and if you ever felt like your memory was impaired during the high, you would be correct, but don’t let your mind wander worrying about your overall brain function: although it may be accurate to feel your memory is impaired whilst high, an eight year study in Australia has found no correlation between marijuana use and long-term memory decline.

First let’s look at what exactly causes memory impairment post-toke. According to an article published in Cell, Volume 155, marijuana increases cyclooxygenase-2 signaling in the hippocampus. The sexy-sounding cyclooxygenase-2 or COX-2 is an an inducible enzyme that converts arachidonic acid (an omega-6 fatty acid) to prostanoids (a subclass of Eicosanoids or signaling molecules that cause inflammation) in the hippocampus.

In short, the increase of COX-2 signaling causes inflammation in the hippocampus, part of the limbic system that deals with the formation of long-term memories.

While it may seem logical that long term marijuana use and subsequent chronic inflammation of the hippocampus could result in cognitive deficit, a study published in Addiction, Volume 106 reveals otherwise.

Data was obtained from 2,404 participants aged 20-24 with 82.3% of participants completing the entire study. The candidates were categorized as light users (smoking monthly or less frequently), heavy (weekly or more often), former or non-users (abstaining for a year or longer). These groups were tested on intelligence and memory three times over the eight year period. They were also surveyed about how their cannabis habits have changed. According to the results:

“…there were no significant between-group differences and only CVLT (California Verbal Learning Test) immediate recall reached adjusted statistically significant longitudinal change associated with changed cannabis use. Specifically, former heavy users improved their performance relative to remaining heavy users.”

This conclusion finds that while heavy users perform more poorly on tests requiring immediate recall, an interpretation reflected in the previous study, the heavy users became indistinguishable from those who had never used if they stopped smoking.