A British study says it provides new evidence that marijuana use may boost the risk that people who struggle with psychosis will relapse.
Losing touch with reality
But critics said the effect seems to be small, and they questioned the validity of the research.
A study co-author stands by the work, however.
"We show that pot use causes an increase in the risk of relapse in psychosis and demonstrate that alternative explanations are unlikely to be true," said Dr Sagnik Bhattacharyya, a reader in translational neuroscience and psychiatry at King's College London.
"It would be appropriate to at least aim for reduction in pot use in patients with psychosis if complete abstinence is not realistic," Bhattacharyya added.
Read: Psychosis is five times more likely for dagga smokers
People suffering from psychosis lose touch with reality and may hallucinate, develop delusions and struggle to think and speak normally. Sometimes psychosis is a symptom of a condition like schizophrenia, bipolar disorder or depression, according to the National Alliance on Mental Illness.
Earlier this year, a British study in the journal BMJ Open linked pot use in psychosis patients to higher levels of hospitalisation and less response to medications. But experts said at the time that this could be because psychotic patients turn to marijuana when they're feeling less stable or because genetics make a person likely to both become psychotic and want to use pot.
In the new study, researchers looked at 220 patients – 90 women and 130 men, aged 18 to 65 – who were diagnosed for the first time with psychosis. Bhattacharyya said his team tried to expand upon the previous research by using statistical techniques to look more closely at the impact of pot.
Read: Marijuana use triggers psychosis
The researchers found that the risk that patients would relapse was 13 percent higher when they used pot than when they didn't. And the odds appeared to go up when the patients used more pot. This suggests – but doesn't prove – that pot use raises the risk of relapse, instead of the other way around, the researchers said.
They added that it's not clear how pot use might boost the risk of relapse. Nor is it clear whether marijuana may have beneficial effects that aren't being measured, or if the drug may cause more relapses while making them less severe.
Mitch Earleywine is a marijuana rights advocate and a professor of psychology at University at Albany, State University of New York. He said the study "suffers from most of the problems that plague a lot of the human research on cannabis and psychosis".
'Ridiculously small risk'
For one thing, he said, the researchers didn't randomly assign one group of patients to use marijuana and another to abstain. Then there's the possibility that a desire for pot is simply a sign that a relapse is coming on, he said.
"In all likelihood, those who notice a need for cannabis earlier might be the same ones who are more likely to have another psychotic break whether they had cannabis or not," he said.
Read: Smoking skunk leads to psychosis
Earleywine also called the increased risk of relapse "ridiculously small".
Study co-author Bhattacharyya disagreed with this assessment. He said it's important that the extra risk seen in his study persisted even after the researchers compensated for factors like higher or lower numbers of psychotic patients who stopped taking their medications or used other illegal drugs.
Efforts to influence cannabis use
Charles Ksir, a professor emeritus of psychology and neuroscience at the University of Wyoming, also criticised the study, saying the increase in risk was small and unproven. "Perhaps the individual begins to decompensate, and as part of that process they are a bit more likely to use cannabis," he said.
What about urging psychotic patients to avoid pot?
"It's fair to say that anyone who has ever had a psychotic break or has a schizophrenic relative should stay away from the plant," Earleywine said.
But Ksir said that "efforts to influence cannabis use among psychotic patients have not been successful in getting them to stop or reduce their use".
The study is published in the journal JAMA Psychiatry.
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