THAT cannabis and schizophrenia are linked is widely accepted. Several studies suggest the drug can set off short-term psychotic episodes in those already suffering from the condition. Other research, though, does more than this. It shows that people with schizophrenia are twice as likely as others to use cannabis. This leads some to argue that the drug is actually a cause of schizophrenia rather than just a trigger—a line of evidence sometimes employed by those who wish to keep it illegal.

But there is another possible explanation for the association. This is that schizophrenics are, for some reason, more drawn to the stuff in the first place. And support for this idea has just been published in Molecular Psychiatry by Robert Power of King’s College, London, and his colleagues. Their work suggests that people who have gene types associated with schizophrenia, even ones who do not have symptoms, are more likely to take cannabis.

Dr Power and his team drew their data from a study of twins in Australia which had asked relevant questions. The first part of their analysis did not depend on the participants being twins. They looked at 2,082 unrelated individuals aged between 23 and 39 who had been questioned by the study’s organisers about their alcohol and illicit-drug use, who had no symptoms of schizophrenia and who had also given blood samples. Specifically, these volunteers had been asked whether they had ever used cannabis, how old they were when they first tried it and how many times in their life they had taken it. Dr Power and his team then analysed the blood samples for genetic markers associated with schizophrenia.

They learned from the interview data that 1,011 members of the sample had taken cannabis, that the average age at which they had started was 20 and that the average number of times they had taken it was 63. The blood analysis let them calculate, from the number of genetic markers each participant had, and the strength of the association each marker had with the development of schizophrenia, a value called the polygenetic-risk score. This ranged from a low of -0.3 to a high of +0.3, values which correspond to a 50% lower-than-average genetic risk of developing the condition and a 50% higher-than-average risk respectively.

From this part of the study the team found two things. One was a correlation between a participant’s risk score and whether he or she had ever taken cannabis. The other was an association between the amount of someone’s cannabis use and those genetic markers most associated with schizophrenia.

The researchers then conducted a second trawl of the data, this time looking specifically at 990 pairs of twins. In this, they found that when neither of a pair had ever used cannabis, which was true of 272 of them, their average genetic risk score was -0.18. When one twin had used the drug but the other had not, which was true for 273 pairs, the genetic-risk score for both averaged -0.02 (ie, almost the same as the general population). When both members of a pair had taken cannabis, which was true for 445 of them, their average score jumped to +0.12.

Together, these findings suggest that people born with a lot of the genetic variations that seem to predispose to schizophrenia are more likely to take cannabis than those born with few of them. The study’s samples are small, so follow-up work will be needed to confirm this result. But if it is so confirmed, that will show the link between cannabis and schizophrenia to be a two-way street. It will not eliminate, as Dr Power is quick to point out, the possibility that taking cannabis increases someone’s chances of becoming schizophrenic. But it will mean that those predisposed to the condition are indeed more drawn to the drug.