The relationship between marijuana and memory is — wait, what were we talking about again?

Oh right, marijuana and memory. There are dozens of studies exploring how ingredients in cannabis affect short-term and long-term memory, but many contradict each other, making it difficult to reach a definitive conclusion.

Here's what we know.

The main psychoactive ingredient in marijuana, THC, does appear to impair short-term memory. It can make it difficult to recall things you learned while high and form new memories. A 2016 study published in JAMA Internal Medicine also found that long-term marijuana users scored lower on verbal memory tests than non-users.

That said, the impairment does not appear to be permanent, according to a 2013 study published in the Annual Review of Psychology. After studying the "working memory" of 44 adults (22 of whom reported heavy marijuana use) over the course of three years, researchers at the University of Amsterdam determined that there was no qualitative difference between the memory functioning of users and non-users.

"If we talk about the direct effect of marijuana on memory (and cognitive functioning in general) then we know for sure that being stoned affects your memory, attention, reaction speed and other cognitive functions," Dr. Janna Cousijn, the lead author of the study, told ATTN:. "An important question you can ask is also to what extent the effects last. My view, reviewing the limited literature to date, is that there is partial, if not complete, recovery after abstinence."

The impact of marijuana on memory depends on dosing and individual users' tolerance to THC.

The more potent a strain of cannabis (i.e. the higher the concentration of THC present in the strain), the greater the effect on a user's memory, a 2006 study published in the journal Psychopharmacology found. Inhaled marijuana also appears to impair short-term memory more than taking cannabis in the form of an edible.

Interestingly, the study also found that heavy cannabis users seem to be less affected by THC in terms of memory functioning than infrequent users. That's likely because heavy users develop higher tolerances to THC and are therefore less sensitive to the psychoactive effects of THC.

Marijuana doesn't cause permanent damage to memory.

Some opponents to marijuana legalization cite studies showing that cannabis negatively impacts memory as evidence supporting their opposition to reform. The idea is that permitting people to use cannabis would cause irreversible damage, but that point has been disputed by recent research, including a 2001 study published in the journal Archives of General Psychiatry.

Researchers at Harvard Medical School surveyed three groups as part of a study looking at the neuropsychological effects of marijuana on long-term users. There were 63 current heavy marijuana users, 45 former heavy marijuana users, and 72 control subjects who hadn't used marijuana included in the study. What they found was that "[s]ome cognitive deficits appear detectable [among heavy marijuana users] at least 7 days after heavy cannabis use," but that those deficits "appear reversible."

Marijuana can have positive effects on memory, too.

For all of the concerns about the effect of marijuana on memory, one part of the research into the subject tends to be left out in conversations regarding drug policy and science: marijuana can help people forget traumatic memories, which could serve a therapeutic function for people suffering from PTSD.

Because ingredients in cannabis respond naturally to the human endocannabinoid system — receptors throughout the brain and body that regulate mental and physical processes, including memory — using marijuana has actually been shown to help facilitate the extinction of negative memories, Nature reports.

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