In what is sure to be a controversial finding among cannabis users and proponents, a review of existing research published this week in The Lancet Psychiatry suggests that a single dose of THC may induce a variety of psychiatric symptoms associated with schizophrenia and other psychiatric disorders.
According to a news release issued by The Lancet on March 17:
A single dose of the main psychoactive component in cannabis, tetrahydrocannabinol (THC), can induce a range of psychiatric symptoms, according to results of a systematic review and meta-analysis of 15 studies including 331 people with no history of psychotic or other major psychiatric disorders, published in The Lancet Psychiatry journal.
The study was funded by the Medical Research Council and was conducted by researchers from Kings College London, South London and the Maudsley NHS Foundation Trust, Imperial College London, Leiden University Medical Hospital, Yale University School of Medicine, Connecticut Mental Health Center, and VA Connecticut Healthcare System.
The study also notes that these psychiatric symptoms are not associated with cannabidiol (CBD), one of the other major active compounds in cannabis. The authors reviewed four studies examining CBD's effects on the development of the same psychiatric symptoms, and no significant differences were found between the effects of CBD and the effects of a placebo. "In studies that focused on whether CBD counters THC-induced symptoms, one study identified reduced symptoms, using a modest sample, but three larger studies failed to replicate this finding."
The aforementioned news release quotes King's College professor Oliver Howes as saying, "As the THC-to-CBD ratio of street cannabis continues to increase, it is important to clarify whether these compounds can cause psychotic symptoms. Our finding that THC can temporarily induce psychiatric symptoms in healthy volunteers highlights the risks associated with the use of THC-containing cannabis products. This potential risk should be considered in discussions between patients and medical practitioners thinking about using cannabis products with THC. This work will also inform regulators, public health initiatives, and policy makers considering the medical use of THC-containing cannabis products or their legalisation for recreational use."
There's an important distinction to note here. Although the researchers found that a dose of THC—which they say is roughly equivalent to a single joint—can induce symptoms that mimic those of certain psychiatric disorders, THC does not in fact cause said disorders in users.
This will come as little surprise to cannabis users, who are well aware from decades of anecdotal evidence that smoking a joint can make some people a little paranoid, but it has certainly never made anyone schizophrenic.
To put things in perspective, consider that in a commentary he wrote for the Straight last August, author and activist Dana Larsen noted that "every analysis of relative drug harms lists cannabis as one of the safest psychoactive substances there is."
You can read the paper, which is title "Psychiatric symptoms caused by cannabis constituents: a systematic review and meta-analysis", at the Lancet Psychiatry website.