This is a complicated question, and one that science has been trying to answer for decades. However, it’s also a question that overlaps heavily with the general study of cannabis and its effect on mental health. This makes it an area of research that is muddied by bias and politics. It’s arguable that a clear answer doesn’t even exist yet, as we are still far from having all the facts.
It is interesting to note that in a book written in 1980, High Culture: Marijuana in the Lives of Americans by William Novak, the author states “bad trips on marijuana are statistically minuscule, but they do occur—especially the first time…But the vast majority of first-time experiences are either neutral or pleasant”. While negative first-time experiences are certainly still in the minority, the sheer number of modern reports implies that some increase in their incidence may be occurring. After all, most regular smokers today know at least one or two people who “couldn’t handle” their first time. This phenomenon may correspond to the increase in THC relative to CBD and other cannabinoids and terpenes that has been occurring in commercial cannabis varieties over the last few decades. Or it may simply be a result of residual chemicals present in poorly-grown cannabis. The market for cannabis in the Western world has so decisively shifted from imported, outdoor-grown varieties containing relatively little THC (and few pesticides, if any) to indoor crops grown with commercial nutrients and chemicals. With most plants, there are specific chemicals and pesticides to be used and that use is dictated by the governing body’s regulations, such as those from the Environmental Protection Agency in the USA (at a federal level). But at least in USA’s case, that doesn’t apply to cannabis since it’s still illegal on a federal level.
Therefore, those growing cannabis have no clear guidance on what can or should be used and when. No such oversight likely means at least some of today’s cannabis in the Western world are more contaminated with chemical residue. There’s also the fact that relative THC content has increased over these last few decades. THC content has risen dramatically in more recently developed strains in much of the Western world and more and more people are getting access to these high-strength strains. We’re now hearing about strains that have up to 40 percent THC. In 1980, levels that high were unheard of with the average THC content being less than 10 percent. Nowadays, average THC content isn’t 35 percent, but it’s certainly higher than the 1-10 percent range. In 2008, the UNODC stated that average content was approximately 10 percent. In Colorado in 2015, the average was apparently more like 18.7 percent! THC really does seem to cause short-term psychosis. We have so much evidence connecting THC with short-term psychotic effects that it’s fatuous to ignore it. While we have little reliable evidence that it causes long-term psychiatric illness, we certainly do have evidence that acute administration of THC causes a state comparable to psychosis in the short term. It’s likely that some of the more susceptible individuals among us (who may be more susceptible due to genetics, state of health, or various other factors) can experience a THC-induced psychosis-like state, which may persist for some time. For most of these people, this state will eventually go away. For a small subset of them, this THC-induced state may trigger an underlying mental illness. This is not the same as THC itself causing the mental illness, as they would probably become mentally ill without any cannabis use. The cannabis use could speed up or possibly exacerbate its onset, though. So while THC shouldn’t currently be blamed for causing mental illness, its short-term psychosis-inducing effects are extremely important to study. From 1972, an Iranian report on narcotics highlights a case of a policeman with no previous history of psychosis who “went into a very violent excitement with paranoid delusions, struggling to get hold of his rifle to shoot his imaginary persecutors” after “a bout of bhang drinking”. Of course, this was during a time of intense controversy on recreational use of cannabis. A time with plenty cases of wild propaganda and unfounded statements about cannabis use (ever see the movie Reefer Madness?!). So how much truth there is in this story we may never know for sure. Then in 2005, we have two case studies of “cannabis acute psychosis”.
Two “regular but occasional” users experienced “depersonalization, paranoid feelings and derealisation” after oral administration of THC. Another 2005 study states “even the critics have accepted that psychotic symptoms can be induced by cannabis, and that such symptoms generally wear off quickly and with complete remission”.
However, this study did find a very strong association between cannabis psychosis and later development of paranoid schizophrenia, backing up the concept that cannabis psychosis can act as a trigger for underlying conditions. In 2009, an excellent review on the existing literature on cannabis and acute psychosis was published, which states “generally these psychotic symptoms are transitory (minutes to hours) but there have been a few reports of symptoms persisting for weeks (…) severe or persistent psychotic reactions are rare, and are more likely to occur in individuals with a pre-existing psychiatric condition”. Again, it is crucial to bear in mind that these persistent negative effects are unusual, and that most people have a pleasant first time using cannabis. Furthermore, even if you find yourself experiencing feelings like those described herein, it is important to try to remain calm and rationalize your experience. Feelings of anxiety, paranoia and depersonalization in first-time cannabis users are usually temporary.