Does marijuana smoking cause psychosis? Or could psychosis drive pot smoking?
If you believe the panned and parodied 1936 film "Reefer Madness," smoking weed will make one crazy and drive you to a life of crime.
Medical science has taken the question seriously, however, and found a strong link between schizophrenia symptoms and cannabis use in many large-scale, peer-reviewed studies. Data from four such studies suggest that cannabis use doubles the likelihood of developing a psychotic illness later in life.
But a few small studies have flipped the direction of causation, suggesting that a genetic predisposition to schizophrenia (a disease marked by psychotic episodes such as hallucination) is itself a risk factor for smoking pot. That might explain why pot use is perennially high (pardon the pun) among those diagnosed with schizophrenia.
A European research team led by Kings College, London, suggests that at least part of the reason why schizophrenic symptoms and pot smoking overlap may lie in shared genetic markers. Their study was published online Tuesday in the journal Molecular Psychiatry.
The results could influence debate over the legalization of marijuana for medical and recreational use.
Researchers studied 2,082 people, about 49% of whom reported having smoked pot. They analyzed the participants' genome, looking for known variations that have been associated with schizophrenia. They found that those with the strongest genetic profile for schizophrenia risk also were more likely to use cannabis, and to use it in greater quantities.
Madness, they suggest, may lead to reefer.
The researchers do not rule out an independent path linking marijuana use to subsequent psychotic episodes, but suggest the overlap between the two is at least partially a two-way street.
"We know that cannabis increases the risk of schizophrenia," lead author Robert Power, a psychiatry researcher at Kings College, said in a statement. "Our study certainly does not rule this out, but it suggests that there is likely to be an association in the other direction as — that a predisposition to schizophrenia also increases your likelihood of cannabis use."
The authors caution that the directional link between reefer and madness may lie in a mix of genetic risk and environmental factors.
There could be other confounding factors in the study as well, they acknowledge. Researchers relied on a previous genome-wide risk assessment that identified slight genetic variations associated with schizophrenia. But that study's schizophrenia sample was likely to have unintentionally included many more pot smokers than would be expected in the general population, and thus may have lumped some genetic markers for pot-smoking with those associated with schizophrenia, the authors note.
In other words, there may have been some reefer mixed with their madness.