In letters to The Lancet Psychiatry, James Coyne and others criticize a UK study for deliberately trying to be politically manipulative. The study made global headlines this year with a finding that smoking high potency marijuana tripled the risk of developing psychosis and schizophrenia.

The study was previously reported on by Mad in America.

Like other correspondents, Coyne argues that there could have been some other underlying factor that led to people at risk of psychosis being drawn to high potency marijuana, which the authors failed to discuss.

“Scaremongering headlines in the media predictably followed,” writes Coyne. “The Daily Mail screamed ‘Scientists show cannabis TRIPLES psychosis risk: Groundbreaking research blames ‘skunk’ for 1 in 4 of all new serious mental disorders.’ Undoubtedly, politicians and policy makers who have already made up their minds about regulation of cannabis will seize on the study as support for their views. The authors must share some of the blame for misinterpretations of their results. They were pre-committed to causal language and failed to acknowledge important limitations of their study… Awareness of the obvious political and policy implications of results, and the likely misuse to which they could be put, apparently failed to discourage the authors from inappropriate causal inferences and ignoring of obvious limitations of their study.”

Coyne also points out that, “Cases and controls were poorly matched. Half of the cases were black individuals and only a third were white, with these proportions reversed among controls. There are also highly significant differences in gender, education, and ever being employed.”

In their reply, the authors of the original study argue that, in the kind of study they were doing, “one generally assumes causality rather than sets out to prove it.” They also state that they did not find any higher “genetic” risk for schizophrenia in the people who developed schizophrenia; therefore, they argue, that proves there was no predisposing factor involved.

Coyne, James. “Cannabis and Psychosis.” The Lancet Psychiatry 2, no. 5 (n.d.): 380–81. Accessed May 6, 2015. doi:10.1016/S2215-0366(15)00113-3. (Full text with free registration)

Gage, Suzanne H, Marcus R Munafò, John MacLeod, Matthew Hickman, and George Davey Smith. “Cannabis and Psychosis.” The Lancet Psychiatry 2, no. 5 (n.d.): 380. Accessed May 6, 2015. doi:10.1016/S2215-0366(15)00108-X. (Full text with free registration)

Crow, Timothy J. “Cannabis and Psychosis.” The Lancet Psychiatry 2, no. 5 (May 2015): 381–82. doi:10.1016/S2215-0366(15)00167-4. (Full text with free registration)

Di Forti, Marta, Evangelos Vassos, Michael Lynskey, Morgan Craig, and Robin MacGregor Murray. “Cannabis and Psychosis – Authors’ Reply.” The Lancet Psychiatry 2, no. 5 (n.d.): 382. Accessed May 6, 2015. doi:10.1016/S2215-0366(15)00177-7. (Full text with free registration)