Cannabis use produces variable effects in different users. Because cannabis can be considered a major public health problem when used frequently, and yet has now been legalized in some places for medicinal purposes, it would be important to know what characteristics of individuals make them more vulnerable to adverse subjective effects of cannabis, the so-called “bad trips.” Family history of psychiatric illnesses was ascertained from family member interviews in order to determine if a genetic predisposition could determine the types of effects one gets when using cannabis. Since major depression is a biologically-based disorder with a genetic predisposition, any correlations with a history of depression may make one more biologically vulnerable to becoming depressed when using cannabis. However, having been prone to major depression did not appear to be associated with becoming depressed with cannabis use, while having a family history of bipolar disorder did. Interestingly, a history of past major depression was associated with not only the “relaxed” factor, but also with hallucinations and perceptual alternations. Although having a family history of schizophrenia was associated with increased irritability and intensity, it was not associated with hallucinations or having paranoia and other delusions. While interesting, our failure to find this association may be due to the limited power of this study (having only 10 probands with a family history of schizophrenia). Sex or ethnicity of the individual did not appear to determine what types of effects of cannabis were reported. An explanation for these associations or lack of is not clear and will require replication in larger studies.
The earlier the age of onset of cannabis use, the more likely one experienced delusions or paranoia. One might speculate that this effect may have to do with the level of brain maturation at the time of onset of use and how much cannabis can then alter the course of that development.
Twenty-six percent of individuals in this study met lifetime criteria for social anxiety disorder, a similar association having been reported by [13–15]. Perhaps this personality trait may lead people to use cannabis so that they can feel more relaxed in social situations, although we do not have an estimate of the percentage of non-cannabis using persons who also exhibit social anxiety and this may simply be a common trait in general.
Barkus and Lewis  reported that schizotypy was associated with psychosis-like effects of cannabis and proposed that people with schizotypy might be more vulnerable to developing a psychosis when using cannabis. The current study fails to confirm this finding. People with schizotypal personality disorder or any schizotypal traits did not more frequently report any evidence of psychosis than those who did not have these traits.
Limitations of this study include (1) many of the variables ascertained were provided by self-report by each subject studied and thus relying on not only retrospective recall of events, but truthfulness of each individual, which is an important drawback of this study. Only a prospective longitudinal study with objective measures observed by a third party would be more definitive. And (2), that our numbers for those individuals whose family members reported major mental illness among relatives were small and thus our negative findings are not definitive and need to be confirmed in larger studies.
In summary, we were unable to show that having a family history of a psychotic illness and thus presumably carrying genes for these illnesses could lead to a tendency to have symptoms characteristic of these illnesses when using cannabis. This could simply be due to the small N of those with family history of mental illness, or could be an initial important finding, given that there is much controversy about whether cannabis itself can cause schizophrenia and has long been considered an environmental risk factor for developing this illness . Larger studies will need to be performed with greater power to see if this finding is confirmed. Nevertheless, it is notable that there are many different effects of cannabis when used as a recreational drug and only 20% of the participants in this study felt “euphoric and happy” with its use, while 40% reported feeling paranoid. These variable effects of the drug and their frequency should be considered as cannabis now has become legalized in several states in the USA and world-wide for a wide variety of medicinal purposes.