Your brain is mushy

Smoking weed is on the brain

Some do it daily, some even several times a day. There are stoners for whom the joint is as natural as the butter on bread or coffee with a cake. The brain is repeatedly flooded with THC, the main active ingredient in cannabis. This is especially true when using highly potent cannabis strains. However, studies indicate that cognitive performance is suffering from the THC tsunami. What does that actually mean in concrete terms? Does weed make you stupid? And can the gray cells recover after stopping consumption?

Image: fotosipsak /

“A phenomenon that every stoner knows. The evening was discussed at the highest level. The world was discussed in its principles. Every sentence, every word, was filled with wisdom. The next morning everything is forgotten. ”The author Jacob Bär, who has made his name anonymous for understandable reasons, describes self-critically in the magazine Fluter his experiences with smoking weed, which he finally gave up because the bad experiences got out of hand.

It is well known that short-term memory is severely impaired when intoxicated with cannabis. When stoned, users sometimes forget what they said five minutes ago or don't know what they wanted to say in the middle of a sentence. Under the influence of THC, the memory takes a kind of time out. This state can definitely be perceived as funny by consumers as long as the “being high” persists. In the natural language of science, which is rather humorless, this condition is also referred to as "situation-inappropriate joking", that is, laughing for no reason - at least for outsiders. But the fun stops when the brain has not recovered properly even after days, because the consumption takes place daily and the thinking apparatus is slowed down again and again by THC. In recent years, science has also been able to provide evidence that long-term smoking affects memory.

Smoking weed makes learning difficult

The Greek researcher Lambros Messinis and his team from the University Hospital Patras, for example, compared the brain performance of cannabis users with those of an abstinent control group in a study. Half of the cannabis group had consumed four times or more per week for over 10 years, the other half between 5 and 10 years at a comparable intensity. Before the start of the experiment, however, all persons had to take a 24-hour abstinence so as not to falsify the results due to acute cannabis effects. After performing numerous standard tests to determine cognitive performance, it was clear that both consumer groups were less able to remember new words and were slower to recall words than the abstinent control group. They also performed worse than the abstinent test subjects in tests for attention and reaction speed.

In a test to determine the ability to make decisions, the people with more than 10 years of continuous consumption achieved particularly poor scores: their results were 70 percent below the norm. The results of the users with a use experience between 5 and 10 years were 55 percent below the normal value, while the results of the control group were only 8 percent lower. Dr. Messinis sums up: "The longer the test participants consumed marijuana, the more their cognitive abilities were impaired, especially their ability to learn new information".

Effects on everyday life

One could argue that the results obtained under laboratory conditions may have little relevance to everyday life. Catherine Montgomery and John Fisk therefore included the everyday effects in their research. In the British study, in addition to the usual brain performance tests, the subjects also had to fill out special questionnaires in which they provided information about their everyday memory. It was about the small failures in everyday life such as forgetting appointments or other things that you actually wanted to remember. Since self-reporting is such a thing, friends and relatives were also asked to assess how often the test subjects make mistakes in everyday life.

The results provide a two-part picture: In contrast to the Greek study described above, no significant differences between cannabis users and abstinent persons could be found in the brain performance tests in the laboratory. Cannabis use therefore has no influence on the performance of the brain. It should be noted, however, that the consumers of the study carried out in Patras, Greece, had a much longer history of use.

The British study, on the other hand, showed clear effects on the everyday life of the users: The people in the cannabis group admitted significantly more mistakes in everyday life. The self-assessments also coincided with the statements of friends and relatives. In their study, the team of authors argues that cannabis users can concentrate under laboratory conditions and deliver the same performance as abstinent people. In real life, however, they would be more easily distracted and consequently show small memory problems and other failures more often than abstinent persons. "Even if cannabis users have shown normal performance in the laboratory, this does not mean that THC has no effect on the underlying neuronal structures," the team of authors concludes in their study.

Loss under stress

In order to achieve as close as possible to everyday life, the test subjects in a study by the University of Cardiff were not restricted in their consumption. The aim of the study was to examine performance under the conditions of a normal working week. Emma Wadsworth and her team therefore put their test subjects to the test for a week both before and after work with a whole battery of psychological tests. At the beginning of the week, both groups were able to deliver roughly equally good results. Differences only became apparent in the course of the working week. For example, the abstinent persons were able to continuously improve themselves in complex tests in which the speed of reaction is important. The stoners don't. In the tests of memory performance, the results of the cannabis group also deteriorated day by day. This was especially noticeable after work. The research team concludes that cognitive deficits, which can be a result of long-term smoking, are often not as obvious and only show up under stress, i.e. when people feel tired.

Permanent brain damage?

Since brain performance is a kind of indicator of the state of health of our brain, it can be assumed that the gray matter is also structurally damaged by constant consumption. In science, however, there is still no consensus on whether permanent brain damage with persistent cognitive impairment actually occurs. In a meta-analysis from 2003, Igor Grant and his team, after reviewing numerous studies, came to the conclusion that, even in long-term users, there was hardly any evidence of defects after abstinence. The researchers in the study sum up that only a small effect on learning new information was found. In principle, the Hamburg researchers Kay Uwe Petersen and Rainer Thomasius also confirm this in an expert report from 2007. In their opinion, there is no evidence of neurotoxic damage from cannabis use in adults.

This is also supported by the fact that the loss of brain performance, even after years of continuous consumption, tends to return to normal when consumption is stopped. In an extensive study, Harrison Pope and his team were unable to find any deficits in cognitive performance even in long-term consumers after an abstinence phase of 28 days.

Early entry problematic

The situation is different if cannabis use begins during puberty. An Australian working group led by researcher Nadia Solowij has focused on the effects of early entry into cannabis use. In a study with young cannabis users between the ages of 16 and 20, they found that their verbal learning ability is already significantly limited compared to abstinent peers. After an average of 2.4 years of consumption experience, deficits would appear in them, as they only occur in adult stoners after many years of consumption. The American researcher Krista Medina and her team also found evidence in a study that even after a month of abstinence, adolescents still have slight deficits in attention and memory, whereas this was not the case in adults.

A study by Mansar Astari and her team provides indications of what happens in the brain. They examined a sample of 14 stoners who started using it at an average of 13 years of age and who have smoked an average of six joints a day in recent years. With the help of a special form of magnetic resonance tomography, the subjects' brain volume was measured. It was found that the young stoners had smaller brain structures than an abstinent comparison group. A region known as the hippocampus was affected. This area is considered to be an important control center for the transfer of learning content into long-term memory. There was also a dose-dependency: the more cannabis was used in the past, the smaller the hippocampus.

Problems from high potency cannabis

It is possible that the cognitive problems caused by cannabis are exacerbated by the use of particularly highly potent herbs. Specially bred cannabis strains contain higher amounts of THC. Analyzes have shown that, in return, the proportion of cannabidiol (CBD) is reduced. CBD itself does not have any psychoactive effects, but it does soften the effects of THC. In research, the effect of CBD on the possible psychosis-promoting effects of cannabis has so far been discussed. It is assumed that CBD has a protective effect here or that highly potent cannabis, which contains little or no CBD, is associated with a higher risk of psychosis.

Celia Morgan and her research team published a 2010 study in the British Journal of Psychiatry in which they presented evidence that people who prefer high-THC and low-CBD cannabis strains are more prone to cognitive problems. The researchers Cécile Henquet and Rebecca Kuepper therefore suspect in an editorial in the same journal that different cannabis strains may also be responsible for the sometimes contradicting study results.


According to the current state of research, brain performance suffers with increasing duration and intensity of consumption. This is shown primarily in the ability to learn and remember. Long-term smoking weed is particularly noticeable under stress. But those who stop smoking weed are rewarded with rapidly improving brain performance. It has not yet been scientifically clarified whether there are still minor impairments that can be traced back to permanent brain damage.

For consumers who regularly take a joint as a teenager under the age of 18 or even under 16, there is convincing evidence that they may damage their brain structure and thereby permanently limit their cognitive performance. This can have a negative impact on academic and professional careers. It is therefore recommended that young people stop using it or at least reduce it significantly.


  • Asthari, M., Avants, B., Cyckowski, L., Cervellione, K., Roofeh, D., Cook, P., Gee, J., Sevy, S. & Kumra, S. (2011). Medial temporal structures and memory functions in adolescents with heavy cannabis use. Journal of Psychiatric Research, doi: 10.1016 / j.jpsychires.2011.01.004. Summary
  • Bonnet, U., Harries-Hedder, K., Leweke, F. M., Schneider, U. & Tossmann, P. (2004). AWMF Guideline: Cannabis-Related Disorders. Fortschr Neurol Psychiat, 72, 318-329. Summary
  • Fisk, J. & Montgomery, C. (2008). Real-world memory and executice procecces in cannabis users and non-users. Journal of Psychopharmacology, 22, 727-736. Summary
  • Floodlights. Issue No. 37. How are we on it? Drugs theme. notebook
  • Grant, I., Gonzalez, R., Carey, C. L., Natarajan, L. & Wolfson, T. (2003). Non-acute (residual) neurocognitive effects of cannabis use: A meta-analytic study. Journal of the International Neuropsychological Society, 9, 679-689. Summary
  • Henquet, C. & Kuepper, R. (2011). Does cannabidiol protect against the negative effects of THC? The Britsh Journal of Psychiatry, 197, 259-260. Summary
  • Medina, Hanson, Schweinsburg, et al. (2008). Neuropsychological functioning in adolescent marijuana users: Subtle deficits detectable after a month of abstinence. J Int Neuropsychol Soc, 13 (5), 807-820. items
  • Messinis, L., Kyprianidou, A., Malefaki, S. & Papathanasopoulos, P. (2006). Neuropsychological deficits in long-term frequent cannabis users. Neurology, 66 (5), 737-739. Summary
  • Morgan, C., Schafer, G., Freeman, T. & Curran, V. (2010). Impact of cannabidiol on the acute memory and psychotomimetic effects of smoked cannabis: naturalistic study. The Britsh Journal of Psychiatry, 197, 285-290. Summary
  • Petersen, K. U. & Thomasius, R. (2007). Effects of cannabis use and abuse. Expertise on health and psychosocial consequences. A systematic review of the internationally published studies from 1996 to 2006. Lengerich: Papst Science Publishers.
  • Pope, H. G., Gruber, A. J., Hudson, J. I., Huestis, M. A. & Yurgelun-Todd, D. (2001). Neuropsychological Performance on Long-Term Cannabis Users. Arch Gen Psychotry, 58, 909-915. items
  • Solowij, N., Jones, K., Rozman, M., Davis, S., Ciarrochi, J., Heaven, P., Lubman, D. & Yücel, M. (2011). Verbal learning and memory in adolescent cannabis users, alcohol users and non-users. Psychopharmacology, DOI 10.1007 / s00213-011-2203-x. Summary
  • Wadsworth, E.J.K., Moss, S.C., Simpson, Smith, A.P. (2006). Cannabis use, cognitive performance and mood in a sample of workers. Journal of Psychopharmacology, 20, 14-23. Summary