A subsequent study conducted with the Dunedin cohort investigated whether specific genes increase the risks associated with early cannabis use.14 The researchers examined the role of the catechol-O-methyltransferase (COMT) gene, whose link with psychosis has been the focus of many studies. The COMT gene encodes the enzyme responsible for the synaptic metabolism of dopamine. A functional polymorphism of this gene, Val158Met, has been shown to slow the breakdown of dopamine, which potentially increases the risk of psychosis.15,16 The results of the study showed that the presence of the valine polymorphism was not significant unless coupled with adolescent cannabis use.13
Persons with Val/Val or Val/Met genotypes and adolescent cannabis use were at increased risk for schizophreniform disorder (with respective odds ratios of 10.9 and 2.5), while individuals with Met/Met genotypes were not. These findings implicate genetic factors as important contributors to the cannabis-psychosis link, but they are in need of replication.
Impact of cannabis use on the course of schizophrenia
The extent to which cannabis use might alter the clinical course of schizophrenia remains a point of contention within the literature. Intuitively, one may expect cannabis to have a negative impact on the expression and course of schizophrenia. Findings suggest that patients with schizophrenia who use cannabis experience increased psychotic symptoms, are more likely to have relapses, have a greater likelihood of rehospitalization, and experience poorer therapeutic response to antipsychotic medication than patients who are cannabis-naive.17,18 Furthermore, pre-onset cannabis use may trigger an earlier age of onset of psychosis, which is of critical importance given the negative prognostic features associated with earlier onset.19 These effects have been reported to be dose-dependent.
It is interesting to note that other studies have been unable to confirm these adverse findings after controlling for potential confounding factors, which include but are not limited to alcohol and drug use, premorbid functioning, and family history. Moreover, it has been suggested that patients with comorbid cannabis use constitute a clinically distinct subgroup of schizophrenia patients.
In this respect, cannabis use may trigger the onset of psychosis in vulnerable individuals in whom a psychotic disorder otherwise may not have developed. As a result, these patients have a better prognosis, exhibit fewer negative symptoms, have better social skills, and have an enhanced treatment response compared with nonusers. In addition, a recent meta-analysis demonstrated that patients with lifetime cannabis use disorders have superior cognitive function compared with nonuser counterparts.20
These conflicting findings may be due to the varying levels of THC/CBD found in street cannabis. The fact that these constituents have divergent properties may explain the manifestation of different psychological symptoms among users. In fact, CBD may actually attenuate some of the unwanted psychopharmacological effects of THC, because it may have anxiolytic and antipsychotic properties.21 Furthermore, CBD has been shown to have neutral or even procognitive effects.22
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