A new analysis highlights that the debate over marijuana's health risks and benefits is complicated and depends on the active compounds involved.
A review of existing research published Tuesday found that a single dose of the main psychoactive ingredient (THC) in cannabis -- equal to one joint -- in otherwise healthy people, can temporarily induce psychiatric symptoms, including those associated with schizophrenia.
"The first takeaway is that for people in general there is a risk, even if you are healthy and taking a single dose, a one-off, you could have these symptoms," said Oliver Howes, a senior author on the study and molecular psychiatry professor at King's College London's Institute of Psychiatry, Psychology & Neuroscience.
The symptoms "are distressing and could affect your thinking. It's not just something that's going to affect people with a history of mental health problems," he added, noting that having symptoms didn't cause them to have a psychiatric illness."
"They are distressing and could affect your thinking. You might not behave in a safe or rational way. It's not just something that's going to affect people with a history of mental health problems," he added, noting that having symptoms didn't cause them to have a psychiatric illness.
However, the analysis, published in the journal Lancet Psychiatry, found no evidence that cannabidiol (CBD), a non-psychoactive compound in weed that's used in some medicinal cannabis and other products, does induce psychiatric symptoms. Howe said this was "reassuring" but noted that the available evidence was fairly limited.
While they are similar compounds and both found in cannabis, THC (tetrahydrocannabinol) and CBD don't have the same effects. The THC is what you makes you feel "high," while CBD is thought to have benefits without producing the same euphoric feeling.
Howes said that regulators, policy makers and health official needed to take into account the potential risk when considering the medical use of THC-containing products or allowing them to be used recreationally.
"As the THC-to-CBD ratio of street cannabis continues to increase, it is important to clarify whether these compounds can cause psychotic symptoms," he said.
The psychiatric symptoms included paranoia, says Howes, including people thinking others were talking about them or threatening them in some way and hallucinations like hearing voices.
"There are also some people who become withdrawn and lose motivation. And whilst those are less dramatic, they could affect ability to work or drive a car. When you take a one-off dose, they [the effects] are temporary and last a few hours, but if you take it regularly, it could be an enduring consequence."
In the review of available evidence, Howe and the other authors systematically examined 15 studies that took place in clinical settings involving 331 people with no history of psychotic or other major psychiatric disorders.
Howe said one strength of the review was that it focused on the effects of cannabis on healthy adults who didn't have risk factors for psychiatric problems.
"This allows us to really test whether these cannabis components themselves lead to psychiatric symptoms," Howe said.
While previous research has found a link between cannabis use and psychiatric symptoms, it's not been clear whether people are using cannabis to help them deal with existing problems.
While Howe said it was "reassuring" that CBD didn't cause psychiatric symptoms, the review found no consistent evidence that it tempers the effects of THC as some scientists had thought.
"In terms of medicinal use of cannabis, it highlights that we do need to think about the possible risk, specifically from THC. If people are developing medicinal cannabis in plant-based form, they need to think about that. That's not to say it can't be a useful medicine but has to be weighed up against these symptoms," Howes said.
The analysis also suggested that smokers are less sensitive to the effects of THC, but this finding was preliminary. Howe said it could be because tobacco reduces receptors in the brain to which THC binds.
"Tobacco brings its own risk. I would strongly not advise taking tobacco, but it highlights to us something about how tobacco and cannabis components act on the brain and that could be useful in future research," Howes said.
David Nutt, a professor of neuropsychopharmacology and director of the Neuropsychopharmacology Unit in the Division of Brain Sciences at Imperial College London, said it has long been known that cannabis could make people paranoid. Nutt was not involved in the review.
"It's been known for centuries that some people get paranoid on cannabis -- this is not the same as developing a psychotic illness," he said. "One of the main reasons people stop using cannabis is because it makes them paranoid, so it becomes unpleasant."
He said medicinal cannabis containing THC such as the MS drug Sativex, which was the first cannabis-based medicine to be licensed in the UK, used elements that can mitigate the psychotic effects and users can built up to higher doses slowly.
"People who use cannabis recreationally should continue to be careful when using the drug because of the potential harms to mental health which can be associated with the drug," said Dr. Michael Bloomfield, a principal clinical research fellow and Translational Psychiatry Research Group head at University College London, who wasn't involved in the research.
"When it comes to cannabis-based medicines, we need the same levels of scientific evidence as we apply to other medicines -- which means that patients and doctors need well conducted scientific studies that assess both the potential beneficial effects and side-effects for the illness that any particular medicine is supposed to treat."