A SINGLE DOSE of tetrahydrocannabinol (THC) – the main psychoactive component in cannabis – can induce a range of psychiatric symptoms, according to a new study published in The Lancet Psychiatry journal.

Single doses of THC, roughly equivalent to smoking one joint, may induce a variety of psychiatric symptoms associated with schizophrenia and other psychiatric disorders, the study claimed, while adding that the effects are larger with intravenous administration than with inhaled administration. 

The conclusion came from a systematic review and meta-analysis of 15 studies which included 331 people with no history of psychotic or other major psychiatric disorders.

The results from the analysis of another four studies found no consistent evidence that cannabidiol (CBD) induces psychiatric symptoms itself or that it moderates THC’s effects in healthy volunteers. 

The review also found that smokers are less sensitive to the effects of THC, but this finding is preliminary and the study’s authors don’t recommend using tobacco for this purpose.

“Our finding that THC can temporarily induce psychiatric symptoms in healthy volunteers highlights the risks associated with the use of THC-containing cannabis products,” King’s College London’s Professor Oliver Howes said.

“This potential risk should be considered in discussions between patients and medical practitioners thinking about using cannabis products with THC.”

The doses of THC ranged across the studies, and researchers attached scores to the different symptoms people reported when compared with a placebo. 

Compared to placebo, THC was found to induce significantly more severe positive psychotic symptoms such as delusions and hallucinations as well as more general symptoms such as depression and anxiety.

The authors, however, also highlight several limitations to their study such as their results contrasting with that of others. They suggest more work is needed to clarify the effects of THC, particularly at the level of individual symptoms.

Lead author Dr Carstem Hjorthøj – who wasn’t involved in the study – said: “Finally, although THC, alone or in combination with, for example, CBD might have a role in treating certain symptoms, caution should not be thrown to the wind.

As Hindley and colleagues have clearly demonstrated, there are at least transient psychiatric symptoms associated with even relatively low doses of THC. Of course, this result should not be extrapolated as meaning that single doses of THC will eventually lead to schizophrenia or other severe disorders.