Marijuana and Psychosis

A number of my friends and colleagues in London have shown how cannabis (marijuana) may trigger a psychotic illness. A team at the Institute of Psychiatry gave healthy volunteers the active ingredient tetrahydrocannabinol (THC).

They then recorded reduced activity in an area of the brain called the inferior frontal cortex that keeps inappropriate thoughts and behaviors in check. The inferior frontal cortex is part of the self-regulatory systems of the brain that stop us swearing inappropriately, and help us to realize that people are not looking at or spying on us. In other words the inferior frontal cortex checks the environment and stops us jumping to conclusions that could lead to paranoia.

The THC was given to healthy volunteers who had not abused marijuana. The effects were short-lived, but more sever in some people than others, suggesting a genetic vulnerability.

In another study from Yale University, THC was given intravenously. Even at fairly low doses, 50% of healthy volunteers began to show symptoms of psychosis. As expected, people who had a previous history of psychosis were particularly vulnerable to the effects of THC.

A separate study has shown that one of these ingredients – cannabidiol (CBD) – has the potential to dampen down psychotic symptoms, and could perhaps form the basis of new treatments.

Whenever people start talking about the possible association between psychiatric problems and using marijuana, there are always loads of people who protest, “But I’ve used it for years and it’s never done me any harm.”

They may be right, but that objection misses four important points:

1. People vary in their sensitivity to marijuana. Mental illness is the result of both genes and environment. Somebody who is very stressed but has no known genetic predisposition may run into trouble, while somebody with strong genetic loading and a minimal amount of stress can get very ill very quickly. And that stress can be anything from marijuana to abuse. There is also a lot of variation in how people respond to specific types of stressors
2. If people have already had some psychological trouble, marijuana can be like pouring gasoline on a fire
3. The age at which marijuana is used is all important. There seem to be “critical periods” in brain development, when marijuana can be particularly risky. There is a strong correlation between the number of times someone uses it under the age of eighteen and their subsequent risk of developing psychosis later in life. This relationship does not seem to hold with regular cigarettes, alcohol or any of the other street drugs looked at so far, suggesting that this is not just a matter of young people who are self-medicating. We cannot prove “causality” any more than we can “prove” that cigarette smoking causes lung cancer. Trying to prove a single cause for an illness is a tricky business. There are some examples, for instance illnesses caused by a single gene, but they are few and far between
4. The marijuana available today is very different from the anemic material that was available in the 1960s and 1970s. Today’s is far powerful

This research provides the strongest evidence yet that modern marijuana can have a significant impact on the brain. Proving a long-term effect would be extremely difficult: it would be neither ethical nor feasible to stimulate long-term psychosis in volunteers.

But clearly if something has an active effect in inducing the symptoms of psychosis after one dose, it would not be at all surprising if repeated use could induce a chronic problem, particularly if someone is genetically predisposed.