Some 188 million people worldwide use cannabis, a number likely to rise as governments move to legalize its recreational use. Eleven U.S. states have already done so, along with Canada and Uruguay.

To better understand the cause of psychiatric symptoms experienced by some users, researchers in the United Kingdom conducted a systematic review and meta-analysis to investigate the effects of tetrahydrocannabinol (THC) and cannabidiol (CBD), which are the active ingredients in cannabis. The review included studies of healthy people who were administered doses of either THC or CBD alone, THC and CBD in combination (to examine if CBD has a moderating effect on THC), or a placebo. Participants in the studies received the THC or CBD through a variety of methods, including intravenous, oral, and inhalation.

The researchers analyzed the findings of 15 studies of THC administration to determine the magnitude of the effect of THC on positive (delusions and hallucinations), negative (blunted affect and amotivation), and general (depression and anxiety) symptoms. They found that one dose of THC administered to healthy adults can induce positive and negative symptoms, with more severity of positive symptoms, such as psychosis. The effect was greater when THC was administered intravenously than if inhaled.

Interestingly, tobacco use was associated with fewer positive symptoms, and older mean age was associated with greater negative symptoms. The researchers provided a narrative report of the findings of seven studies on CBD. In the four studies that examined CBD administration alone, no significant psychopathological effects were found, nor did CBD have a significant effect on THC-induced symptoms in the three studies that examined coadministration of the two substances. The authors warn that the findings of psychiatric symptoms in healthy adults administered just one dose of THC "highlights the risks associated with the use of cannabis products, which should be factored into risk-benefit discussions between patients and medical practitioners."-Frank Brodhead