Smoking just one joint of marijuana is enough to trigger psychotic, depressive and anxiety symptoms in otherwise healthy people, British researchers report.
The review of data involved 331 people with no prior history of psychotic or other major psychiatric disorders. It found that there's enough THC (tetrahydrocannabinol) — the main psychoactive ingredient in pot — in one joint to cause serious psychiatric 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," said study leader Oliver Howes, of King's College London.
His team also found no evidence that another marijuana component, CBD (cannabidiol), in any way lessened the effects of THC. On the other hand, CBD appeared to play no role in spurring psychiatric symptoms, the researchers said.
Their conclusions were published March 17 in The Lancet Psychiatry and were based on data from 15 previously published studies.
As Howes' team pointed out, the link between marijuana and psychotic symptoms — such as hallucinations and paranoia — isn't new. The first study to find such a connection was published more than 150 years ago.
But the new review is one of the first systematic analyses of data collected since then.
Howes' group studied rates of psychiatric symptoms for people who ingested marijuana in a variety of ways, and compared those rates to those of people who received a placebo. Symptoms such as depression, anxiety, apathy, delusions and hallucinations were included in the study.
On average, the amount of THC ingested equaled that of a normal smoked joint, the researchers said.
The study found an uptick in risk among marijuana users for all the psychiatric symptoms on their list. Factors such as the user's gender or age, or the way in which they ingested marijuana, didn't seem to matter.
The review also suggested that tobacco smokers might be less sensitive to the effects of THC. This finding, however, is preliminary, and tobacco should not be used for this purpose, the researchers said.
In a journal news release, Howes noted that the THC content of marijuana keeps rising, and "as the THC-to-CBD ratio of street cannabis continues to increase, it is important to clarify whether these compounds can cause psychotic symptoms."
It's also an important point for people considering the use of medical marijuana, he added.
"This potential risk should be considered in discussions between patients and medical practitioners thinking about using cannabis products with THC," Howes said.
None of the findings came as a surprise to Dr. Scott Krakower, who specializes in substance abuse treatment for teens and young adults.
Prior studies "have demonstrated that cannabis may be linked to future onset of psychotic and other mental health symptoms," said Krakower, who is assistant unit chief of psychiatry at Zucker Hillside Hospital in Glen Oaks, New York. He said the connection is especially strong for younger pot users, those who use the drug frequently, and people at genetic high risk for psychosis.
"What is especially concerning are the psychotic symptoms, which are least transient in nature, and may be difficult to predict," Krakower said, and "patients may not realize that even a one-time use of marijuana may cause symptoms."