Study Finds No Long-Term Neurological Impairments with Cannabis Use

According to a new study, any neurological effects caused by cannabis use are only statistically significant in the immediate days after termination. Amy M. Schreiner of the Department of Psychology at the University of Central Florida recently led a study that looked at 33 existing meta-analyses of cognitive impairment experienced by heavy cannabis users. The goal of the study was to determine whether the negative effects of cannabis use, such as memory impairment, attention, and executive functioning issues persist after use has stopped. Tetrahydrocannabinol (THC), a main component of cannabis, is believed to be the leading cause of these negative effects. Because THC remains in the fat cells of an individual long after it has been ingested, researchers have questioned whether it can continue to deliver negative influences days and weeks after its ingestion.

Schreiner chose to focus on studies that involved heavy cannabis users who had stopped using for a period of time. Thirteen of her studies included users who had abstained for 25 days, while the others had shorter abstinence periods. This allowed her to review a number of factors contributing to the negative symptoms. For instance, those experiencing withdrawal may have had additional problems with anger, irritability, anxiety, or even depression as a result of stopping their drug use. However, when Schreiner examined all of the variables and reviewed all the studies, she was unable to prove that the residual effects of cannabis caused cognitive impairment.

When Schreiner looked at studies that included heavy cannabis users evaluated during states of extreme intoxication, she found some statistical impairments in memory, learning, and attention. But she was unable to provide evidence of long-lasting impairment. Specifically, the participants demonstrated no significant cognitive deficiencies once the intoxication period ended. The study did demonstrate some negative effects on motor functioning and verbal skills as well, but these were also minimal after intoxication. Schreiner added that even though some of her findings were statistical, they may not have been clinically significant and may have had little or no effect on the practical functioning of the individuals. Additionally, Schreiner found no symptoms of impairment in the individuals who had abstained for 25 days. In conclusion she said, “These results fail to support the idea that heavy cannabis use may result in long-term, persistent effects on neuropsychological functioning.”

Schreiner, A. M., Dunn, M. E. (2012). residual effects of cannabis use on neurocognitive performance after prolonged abstinence: A meta-analysis. Experimental and Clinical Psychopharmacology. Advance online publication. doi: 10.1037/a0029117

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  • MO


    July 10th, 2012 at 3:36 PM

    potheads around the world are rejoicing, you know

  • Larry


    July 11th, 2012 at 4:08 AM

    OK this addresses the mental health issues that have been insinuated to be linked to long term heavy use. How about the physical effects on smokers? Are there any numbers that confirm whether lung cancer rates could be higher for these smokers? I don’t know that even if there was evidence of that that this would be enough to deter young experimenters, but maybe it would make long term heavier smokers think twice about lighting up again.

  • Chris


    July 11th, 2012 at 6:29 PM

    Smoking marijuana does not increase your chances of cancer. In fact, cannabis targets cancerous cells and for a matter of fact, stimulates brain cell growth.

  • Larry kallis

    Larry kallis

    July 11th, 2012 at 9:06 AM

    @MO:Yes,we are, thank you. And why not?! Did we really need this study to tell us what many of us already knew?!

    If a person consumes alcohol and acts differently during the high then does it mean he’s gonna remain the same even once he’s sober? Then why this differential treatment to cannabis and all the biased non-scientific talk that is always going around? there is a short-term problem with memory under the influence of cannabis but it does no harm in the long run. Anybody who knows the ABC of cannabis would tell you that.

    Cannabis is one of the safest things to consume and there’s no denying that.We all know of the lies that we were fed with during the peak of the war on drugs. We now have proof that most of those things were false.

    I’m not saying everybody should consume cannabis or that it should be in the hands of kids and teens. But if I’m a responsible adult trying to consume something that is not harmful to me or anybody else then what right does the government have to stop me, to criminalize the activity and put me behind bars in order to fund their corporate friends?! How is it even a crime when there is no victim?! And if you’re talking about the underground economy due to cannabis, then the government should please do us all and itself a favor and let it come out in the open, decriminalize, or maybe even legalize it!

    It is an absolute irony when you consider the fact that the largest preventable reason for death around the world is your normal cigarette and yet it enjoys government support and we are constantly bullied by big tobacco!

  • Alexandra


    July 11th, 2012 at 11:53 AM

    @ Larry kallis, I think that you raise some very valid points that I am sure that we can all argue to death, but the true fact of the matter is that when you are using something that causes you to feel out of the morm of what most people regard as generally acceptable behavior, it’s probably not a good idea to go around promoting it. Here in the Us, it is illegal, plain and simple. Am I willing to throw caution to the wind and break the law all because there is one study that says cannabis is not harmful in the long run? No because I am sure that I could go out and find 50 other studies which will say that it is. It is all a matter of believing what you want to believe and obviously you ahve found the study that reiterates that which you think is correct. Fine, that’s the right thing for you. But that does not mean that it must be advocated for everyone else as being safe.

  • Soren


    July 11th, 2012 at 12:31 PM

    In response to the second poster, Goggle Professor Tashkin cannabis and lungs. Its eye opening. Not only does cannabis not cause lung cancer it may even protect against getting it. Cheers,

  • PhiLlip


    July 11th, 2012 at 1:39 PM

    as a medical marijuana user, it is heartening to read there are no long term effects from MJ use.

    but that does not mean indiscriminate use for recreational purposes.there is that fine balance between use and abuse that not everyone can maintain.

  • Adam


    July 11th, 2012 at 11:21 PM

    This isn’t the first, or only study that has proven that the things we have been told about marijuana’s possible negative effects were untrue. There’s plenty of scientific research that shows that marijuana is virtually harmless. The choice in this case should be up to the individual. Use and abuse is all about individual responsibility, and if one can’t be responsible for themselves with a substance that has been proven to have very little addictive potential, then they are the danger to society, not the substance they’re using. I think Alexandra is correct about people believing things because it’s what they want to hear, but in this case I don’t agree that we are the ones that are doing it. Nobody said to break the law either, we just think it’s time to change it. we do need this study, because the prohibitionists won’t just take our word for it, but they have no choice when the facts are staring them in the face.

  • Dan


    July 12th, 2012 at 1:36 AM

    most of you cannabis supporters are so naive to think that smoking cannabis is either neutral or beneficial for your health, you are taking hot gasses into your lungs, it really pisses me off.. you obviously don’t know what you are talking about.. do your research before you go spouting stuff like this, @soren – where the cannabinoids in the plant do counter some of the carcinogens also contained in it this is not nearly enough evidence to say something so stupid as it protects you from lung cancer!. @ Larry – do you not realize that the human body retains cannabinoids? so of course there is reason for this type of study. all of that being said, i am 100% for the legalization of cannabis in the UK, but this is the main problem, people on both sides of the argument aren’t educated enough about it, now that is what is preventing the free use of the plant.

  • Tommie


    July 12th, 2012 at 4:27 AM

    I don’t really have an opinion here one way or another, but I do know that I have always questioned why cigarettes are legal, and they have so many negative effects, but marijuana is illegal and does not seem to show anywhere near the negative health outcomes.

  • Adam


    July 13th, 2012 at 6:33 PM

    What arguments have the goverment got to still keep this PLANT ilegal? They can make billions if they sold it, and isnt it the number one crop in calIfornia even tho its for medical use? But it wont stop the dealers selling because the goverment will most likely put a cap on the strengh of the cannabis and we all know its getting stronger all the time.

  • D


    December 9th, 2014 at 4:39 PM

    To those claiming that this is ‘proof’ of anything – it is not. Just because one study shows something does not mean it proves anything or many other studies would have already proven that marijuana is dangerous, cancerous and triggers mental illnesses.

    I’m all for marijuana legalisation, but I can’t help but notice that many of the people I surround myself with are heavy users and I see the varying effects of addiction to marijuana first hand. Many of them do suffer mental illnesses such as anxiety, depression and paranoia and whilst their grades vary from very poor to almost excellent – they all do exhibit memory and motivational problems. They rarely eat – even when they smoke it.

    I may have not read it right, but this study doesn’t explain how long the participants have been using, and I wouldn’t call 25 days and appropriate length to claim ‘long term’ abstinence. It doesn’t look at their neurological levels before use nor explain the extent of the withdrawal symptoms.

    From other research, I have read that heavy use shrinks the area of the brain related to emotion and rewards and motivation.

    Like I said, I am all for legalising weed, but I see the effects firsthand and that makes me hesitant. I’d much rather that weed were able to be grown then eaten. Raw cannibis is supposedly a miracle food. Smoking it for health benefits does sound quite counter-intuitive.

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