Cannabis Causes Schizophrenia: Myth or Truth?
This text has been waiting to be written for a long time. But it was the “provocation” of a group of friends, curious about the topic, that made me organize my ideas and dive into the references to properly explain this very controversial issue surrounding Cannabis. I truly appreciate the “nudge” because I finally broke the inertia and decided to write. I hope you enjoy it!
The question here is straightforward: Does cannabis cause schizophrenia? However, the answer isn’t that simple. And notice that the word “cause” is highlighted here. If the question were “Is there a relationship between cannabis and schizophrenia?” I believe the best answer would be “yes.” However, the way this topic is publicized is almost fearmongering, and highly exaggerated when considering the typical user profile. After all:
- The vast majority of cannabis users do not develop schizophrenia.
- The vast majority of people with schizophrenia are not cannabis users.
Got it? Great, let’s start there.
It has already been relatively clearly demonstrated that the THC in cannabis causes psychotic effects at certain (high) doses. If you expose someone to this experience in a lab, with increased doses, they will gradually show signs of acute anxiety until it develops into psychotic symptoms. It’s the same symptomology that people with acute anxiety crises express; once triggered, the process is the same. What changes here is clear even from the name: substance-induced acute psychosis (hold onto that term, we’re far from talking about schizophrenia).
To me, there’s one study that definitively clarifies the situation. It was led by researchers from the Department of Psychiatry at Oxford University and used the typical rigor of a reliable pharmaceutical study: parallel-group design, double-blind, and placebo-controlled. In this experiment, they selected individuals who had prior experience with cannabis and reported recent paranoid ideation. In other words, it’s a biased population, but intentionally so. They took these individuals and administered THC to them — via injection! 😲 I find it curious that the ethics committee approved this study, but I’m glad they did because it helps clarify this debate a lot.
So, importantly, in practice, the study aimed to understand whether THC triggered acute psychotic crises in high-risk individuals (those with a high likelihood of experiencing such an event), and in this same population, to understand the progression of symptoms, the cause of the crisis, etc. The dose of THC used was 1.5 mg intravenously, which isn’t much, though directly into the vein. It would be roughly equivalent to 15 mg orally or about 6 mg smoked. In summary, it’s close to the experience of smoking a whole joint, of the “common” type found here in Brazil. Since the dose wasn’t very high, they only managed to reach the point of paranoia. I believe that if they had increased the dose, these same individuals would have had a psychotic episode.
In this study, THC unequivocally increased paranoia, caused negative affective symptoms (anxiety, worry, self-deprecating thoughts, and depression/sadness), a series of strange experiences, and cognitive impairment in short-term memory. Here’s the warning: if you have that friend who smokes and gets weird, you already know, it’s a clear sign they are susceptible. The direct cause of paranoia found in this study was mental confusion/strange experiences, mixed with a sudden increase in anxiety. Interestingly, they also found that the intensity of paranoia symptoms was sensitive to an environmental change: reading about cannabis-induced paranoia before the THC injection significantly worsened the symptoms — the individuals became even more paranoid, showing that there is a clear relationship between the psychological effect and the circumstances in which the substance is used. In the field of psychedelics, there’s a lot of talk about “set & setting”, which is nothing more than your internal state and the environment, helping to modulate the experience with the substance.
This other study confirms the findings of the one above in a more naturalistic context, allowing high-risk individuals to smoke “standardized” joints with 5.5% THC and observing their acute reactions. Even more importantly, it confirms that the same substance administered to non-susceptible individuals does not provoke the same sensations. See the graphs below to witness this firsthand.
In reality, the story is different
The point is that epidemiologically, this relationship is controversial, so much so that in a broad population — say, normal society — this relationship is not as direct as in the experiment above. A relatively small percentage of the population is susceptible to paranoia, for instance (thankfully). It’s estimated that around 2 to 4% of people have a so-called paranoid personality that could be considered pathological. (https://my.clevelandclinic.org/health/diseases/9784-paranoid-personality-disorder) These individuals are at serious risk of developing full-blown schizophrenia, as we will see later. The causes of paranoia are diverse, involving some that are genetic/biological in origin, but also circumstantial/environmental. (https://en.wikipedia.org/wiki/Paranoia) About ten times more people report having felt persecuted or having had some “paranoid” experience (British data). But then, well… who hasn’t?
(Learn more here: https://www.livescience.com/3064-freak-paranoia-common.html).
Paranoia is defined as the exaggerated or unfounded fear that others are trying to hurt you. That includes thoughts that other people are trying to upset or annoy you, for example, by staring, laughing, or making unfriendly gestures
Still, in this study that followed 100 individuals at “ultra-high risk” of developing psychosis (already presenting psychotic symptoms), only one-third of them (32%) had a psychotic episode within one year of the study. According to the article, 18% of the individuals reported abusing cannabis. (https://www.ncbi.nlm.nih.gov/pubmed/12406123). If these numbers were extrapolated to a “normal heterogeneous” population, we could estimate that among the 2–4% with psychotic personalities, one in five are problematic cannabis users, and of those, one in three will have a psychotic episode within a given year. In other words, cannabis-induced psychosis is relevant for approximately 0.12% to 0.24% of a given population in a given year, which is the epidemiological parameter we call prevalence. Epidemiologically, it’s not considered rare, but it’s far from common…
Nobody does this calculation. Most people of this generation and the last grew up hearing the “meme” that cannabis causes schizophrenia. This idea has been built up over a long time, and it “sticks” because it’s easy to remember and scary enough to make people believe it will keep them away from this “evil.” Intuitively, our culture associates “scaring” with “inhibiting behavior” — basic child psychology, like telling the story of the boogeyman, the woman in white, or whatever crazy story your parents told you…
Even psychiatrists, who are trained specialists, often make this kind of “direct associative” judgment between cannabis and psychosis because they have contact with a very biased sample of individuals. They suffer from something we call the “consulting bias.” In fact, the individuals they come into contact with in their practice are those who had problems with the substance. So imagine those 2–4% mentioned above: the psychiatrists have access to a fraction of those individuals who smoke cannabis and, at some point in their lives, had an episode. It’s typical exaggerated generalist thinking — they are probably seeing 0.2% of the population and extrapolating to the other 99.8%. It’s like judging a forest by looking at the details of a single tree… this is a well-known cognitive bias.
(https://www.visualcapitalist.com/every-single-cognitive-bias/).
So, to recap, THC can cause paranoia and probably psychotic episodes, yes. The mistake is assuming this is true for all users… it’s actually a relatively small portion, and it can be avoided by taking harm-reduction measures. (On the other hand, even without using cannabis, it’s possible that at some point, the individual will have a psychotic episode for another reason — another substance, acute stress, nutritional imbalance, etc.)
A psychotic episode is a real possibility, but in the cases where the individual presents this problem, it occurs due to an interaction between the individual’s biology (which was already prone) and exaggerated and inappropriate substance use (without safety, vulnerable individual). As we saw above, it’s already known that the context of use greatly influences the occurrence of acute anxiety, which evolves into paranoia and eventual psychosis with increased doses.
The most correct statement would probably be that “substance abuse” increases the chance of triggering a psychotic episode. Certainly, cannabis can provoke this reaction acutely, as we saw above, but in practice, the individuals who go through psychotic episodes tend to have a rather complex history, different from the “laboratory experimentation” reality. Direct extrapolation is very simplistic. This study brings an interesting reflection on the subject. The objective here was to evaluate 404 individuals in their first psychotic episode and try to understand the “trigger” for that episode, as well as the underlying life history that may have contributed to its occurrence.
Note in the table below that approximately 52% of individuals in their first psychotic episode had a history of substance abuse/dependence throughout their lives (last column on the right). Approximately one-third of the individuals reported problematic/abusive use of cannabis, while the same one-third reported problematic/abusive use of alcohol. There is no difference between cannabis and alcohol (about 35% for each). The same proportion of individuals (50%) had no history of substance abuse or dependence and had the psychotic episode nonetheless, which is important to mention. This study reports a relatively high prevalence of substance abuse in this “psychotic” population and seems to emphasize the idea that dependent individuals are susceptible to psychotic episodes and tend to present more severe symptoms. This is quite different from claiming that every user will eventually have a psychotic episode, isn’t it?
Are psychosis and schizophrenia synonyms?
Now, a very important point: just as having a seizure does not necessarily mean someone is epileptic, having a psychotic episode is different from being schizophrenic. People often confuse the two because psychosis is one of the typical characteristics of schizophrenia — that stereotypical image of someone hallucinating, completely detached from reality. Although psychosis is part of the schizophrenia picture, it’s not the whole story — psychosis and schizophrenia are not synonyms. The table below summarizes the rest of the symptomology.
That said, there is a chance of transitioning to schizophrenia in individuals who have experienced a psychotic episode, so it’s something to be aware of. It’s a “yellow-orange” alert, I’d say — more than yellow, and almost red — that the individual could enter a chronic state, i.e., become schizophrenic. And that’s very serious.
In this UK study that followed 3,486 patients over 15 years in hospitals, they concluded that about 17% of individuals who were admitted with substance-induced psychotic disorders indeed progressed to a full diagnosis of schizophrenia. This progression occurs over an average of 13 years, meaning it’s a very gradual process. But for a significant portion, the progression occurs within five years (80% of cases) or even faster — in just two years for half of the individuals.
This rate of progression is the same, regardless of the substance that caused the psychotic episode. There was no difference between individuals who were hospitalized due to a cannabis-induced episode, cocaine, opioids, sedatives, stimulants, solvents, or a mix of all these, although — truth be told — alcohol is substantially below all the others, along with hallucinogens. If I were to write a headline based on evidence, it would be more honest to warn that “Cocaine causes schizophrenia” than to blame the usual suspects, cannabis and LSD. Right?
One interpretation of this study is very important, see below. In a loose translation: “Cannabis use can be a risk factor for developing psychotic disorders in vulnerable groups, including individuals who used cannabis during adolescence, those who have had any psychotic symptoms, or those with a high genetic risk for developing schizophrenia. However, in this study, we identified that cannabis-induced psychotic disorder was not an independent risk factor for the development of schizophrenia.” See for yourself below.
(https://www.ncbi.nlm.nih.gov/pubmed/28464965)
In another, even larger study with over 18,000 patients from Finland, the conclusion was somewhat different. Over eight years, 46% of individuals who were hospitalized for cannabis-induced psychosis progressed to schizophrenia, while 30% of individuals with amphetamine-induced psychosis were diagnosed with schizophrenia. It’s quite high. The authors discuss methodological differences that result in overall higher conversion levels in the Finnish study, but still, it’s quite relevant and something to watch out for. In practice, if you’ve reached the point of having a psychotic episode, stop everything, please. In my opinion, it’s actually best to stop much earlier because the signs are gradual. Full-blown schizophrenia doesn’t develop overnight. And it all starts, remember, with anxiety, paranoia, and strange ideas when under the influence of cannabis. If you experience this, it’s a yellow alert.
Particularly, there are situations that pose even greater risk. For example, the use of edibles (cakes, brownies, etc.) presents a huge risk due to the difficulty of dose adjustment. It’s common for individuals to “overdo it” when eating, generally due to inexperience but also because it’s hard to know exactly how much is in the product (in the pharmaceutical industry, we would call this a problem of dose uniformity). This occurs even in countries where recreational use is regulated, as oral absorption of cannabinoids is also quite erratic. In these cases, the probability of an accident like the ones mentioned above is higher. Ironically, smoking/vaporizing may be a safer form of use for the individual, due to the ability to titrate (adjust) the dose more finely and individually.
“In practice, if you’ve reached the point of having a psychotic episode, for the love of God, stop. In my opinion, it’s actually best to stop much earlier because the signs are gradual. Full-blown schizophrenia doesn’t develop overnight.”
And now, what to do during a psychotic crisis? A complex and difficult question, and one that is beyond the scope of this post. My best answer if you observe someone going through this is to seek an emergency room because there are important palliative measures to be taken.
If you’ve made it to this post because you’re a user and curious about the topic, my suggestion is to inform yourself about “harm reduction” in cannabis use and stay alert for any of the signs mentioned above. If you’re in a high-risk group or exhibit symptoms of paranoia or psychosis, the responsible action is to stop using. If you’re a mental health professional, see the next section in this text.
This topic was already covered in another post, from which I took the table below. If you’re interested in learning more about the risks associated with cannabis use, it’s worth a read.
Guidance for Professionals
If you’re a mental health professional, I recommend the literature below to learn more about the topic and be well-prepared for cases like these. This article provides detailed information about appropriate measures in the case of acute cannabis intoxication. In a word: clozapine, but please read the whole article.
Below are 3 very important references on clinical procedures, aimed at doctors and other health professionals.
Case study reference: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4427213/
This is a very detailed review, worth reading: https://www.bmj.com/content/340/bmj.c1571
This paper provides very clear guidelines on what to do:
https://www.semanticscholar.org/paper/Comorbid-Cannabis-Misuse-in-Psychotic-Disorders-%3A-Raby/50236a12b1d860ba1ab21903f4cf05852b82e2a6
To wrap up, I think it’s worth posing a provocation. As I’ve extensively commented in this post, psychosis can be induced by several substances, though the historical stigma has fallen on cannabis. To illustrate this with a bit of irony, below is a very curious account of a manic-psychotic episode caused by an excess of Ginseng tea.
It’s documented — there was a clear association from a clinical perspective. And theoretically, ginseng is something that’s available in any supermarket. A huge risk to the population. Of course, the intention here is to stir things up; I’m being ironic, but it’s worth reflecting: if the intention behind substance prohibition is to protect individuals, shouldn’t we ban ginseng, or at least regulate its sale to teenagers, perhaps limiting the amount? Should we ban ginseng because it “causes schizophrenia”?
I think not (obviously), but taken to the extreme, this is the same reasoning being applied to cannabis, and that’s one of the strongest arguments against it — one that frequently arises when there’s nothing else left to say in a discussion: “but it causes schizophrenia.” It’s the “killer” argument for anyone who believes prohibition is a correct and effective measure. Something to think about.
In my opinion, “Prohibiting is laziness to educate” as I once heard from a friend. And it makes sense. But to clarify: I don’t consider cannabis to be a plant devoid of risks, not at all. The risks exist, and they are real. Adults who choose to use it should do so responsibly, seek education on the subject, and be fully aware of their own actions and consequences.
It can cause the problems mentioned above, and others mentioned in this post (cognitive impairment, lack of motivation), though again, this is not a generalized truth for all individuals, and these symptoms cease when use is stopped.
When I hear laypeople say this, I don’t judge, “it’s okay,” because these are the simple-to-digest pieces of information that reach them. “Cannabis causes schizophrenia” is a meme, easy to understand/remember. Understanding the situation in a slightly more complex way, as I’m explaining here, requires a deeper understanding of the phenomenon, plus professional experience.
Now, when a professional in the field continues to claim this type of information, in my opinion, something is wrong.
- They suffer from a sampling bias (psychiatric consultation) and are unaware of it because they don’t know users in other contexts, thus making a simplistic direct association.
- They believe they are superior and don’t really want to inform/educate people because they think they won’t understand. So, they believe “scaring” is an effective way to curb use, which they see as harmful.
- It’s pure intellectual dishonesty. They’re manipulating public opinion, reverberating the idea that every user is sick and needs to be hospitalized, often with a conflict of interest.
And how does this affect the medicinal use of cannabis?
Good question. In a therapeutic context, these risks are mitigated by dosage, product type, and close monitoring. From my perspective, the cost-benefit ratio has been favorable in most cases where efficacy is obtained.
The main goal of this Medium is to provide high-level information about science and technology in the realm of medicinal cannabis, a field of medicine that has been flourishing in recent years. Interested? Keep following along!
About the author — Leading the Way in Medicinal Cannabis Innovation
Fabricio Pamplona is a Brazilian scientist and entrepreneur, with a Ph.D. in Pharmacology, recognized internationally for his groundbreaking work in the medicinal use of Cannabis derivatives. Ranked among the top 10,000 most influential scientists in Latin America, he stands out as a thought leader in the field. Pamplona has played a key role in the development and launch of six market-ready products and currently serves on the board of three biotech and health-focused companies.
In recent years, he has shared his expertise on prestigious stages such as Harvard, TEDx, and SxSW, while also serving as a curator for the PATH Festival and an advisor to Brazil’s National Council for Scientific and Technological Development (CNPq). His journey is driven by a deep belief in innovation, turning scientific curiosity into real-world solutions that benefit society.
Join the conversation on Medium to explore how science, entrepreneurship, and Cannabis converge to transform the future of health.
References, because speaking off the top of my head just won’t do!
https://academic.oup.com/schizophreniabulletin/article/41/2/391/2526091
https://en.wikipedia.org/wiki/Set_and_setting
https://my.clevelandclinic.org/health/diseases/9784-paranoid-personality-disorder
https://www.livescience.com/3064-freak-paranoia-common.html
https://www.visualcapitalist.com/every-single-cognitive-bias/
https://www.ncbi.nlm.nih.gov/pubmed/12406123
https://www.ncbi.nlm.nih.gov/pubmed/28803095
https://www.ncbi.nlm.nih.gov/pubmed/12537033
https://www.ncbi.nlm.nih.gov/pubmed/28697856
https://www.ncbi.nlm.nih.gov/pubmed/28464965
https://www.ncbi.nlm.nih.gov/pubmed/23419236
http://primarypsychiatry.com/wp-content/uploads/import/0409PP_Raby.pdf
There are several guides on the internet about harm reduction in cannabis use: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.202.4159&rep=rep1&type=pdf
