Richard G. Petty, MD

Cannabis And Mental Illness

The debate about a possible relationship between smoking cannabis and developing mental illness – in particular schizophrenia – has been going on since the 1960s. For a long time it looked to many people as if "Cannabis psychosis" was a myth. But I’ve never been so sure. I have seen too many people who have smoked a lot of cannabis and then become psychotic.

The whole issue is now becoming clarified. We have known for years that cannabis can precipitate psychosis in people already suffering from mental illness, but over the last five years, a series of papers  (article 1, article 2, article 3there are many more) from Scandinavia have looked at young army conscripts and found an association between the number of times that they had used cannabis and their subsequent risk of developing schizophrenia. If they had smoked more than 50 times by the age of 18, their risk of developing schizophrenia was as much as six times higher. A new study from Cambridge in England has found that repeated use in children is also associated with a 2-3 fold increased risk.

I once talked about some of this data at a meeting in Northern California and had a "vigorous" debate with some indignant colleagues who claimed that cannabis was perfectly safe. Perhaps it is in well-adjusted adults, but I’m not so sure.

And this new research raises a number of important points:

1. The cannabis that is now used by young people is much stronger than that which was on offer in the 1960s and 1970s, and is sometimes also adulterated with other substances.

2. The age at which cannabis is smoked appears to be crucial: the data suggests that it is a problem if used during the vulnerable period of brain development that occurs during early and mid-adolescence.

3. Can we say that the cannabis is having a causal role in triggering mental illness? The answer is that we can no more prove it than prove that smoking causes lung cancer. In my book Healing, Meaning and Purpose I discuss the myth of "uni-causality," the idea that there is one cause for an illness. Apart from trauma, there are extremely few examples of one illness being caused by just one deranged gene, one missing nutrient or one external toxin. There will likely be genetic, social and environmental factors that will together determine whether or not cannabis could cause psychosis.

4. Could this just be self-medication? People taking cannabis to try and treat their symptoms? That is possible, though we then have to ask why we are not seeing a similar relationship with any other substances like alcohol or Ecstasy.

5. Finally, there is some recent evidence that one of the key active ingredients in cannabis, tetrahydrocannabinol (THC), can disrupt the normal development of the microtubules that guide the development of neurons in some regions of the brain.

The moral of the story? Cannabis isn’t good for you, and it can be REALLY BAD for people during the vulnerable period of brain development.

Addendum Dec. 22, 2005:  Hot off the presses!  Another article on cannabis and schizophrenia. 

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About Richard G. Petty, MD
Dr. Richard G. Petty, MD is a world-renowned authority on the brain, and his revolutionary work on human energy systems has been acclaimed around the globe. He is also an accredited specialist in internal and metabolic medicine, endocrinology, psychiatry, acupuncture and homeopathy. He has been an innovator and leader of the human potential movement for over thirty years and is also an active researcher, teacher, writer, professional speaker and broadcaster. He is the author of five books, including the groundbreaking and best selling CD series Healing, Meaning and Purpose. He has taught in over 45 countries and 48 states in the last ten years, but spends as much time as possible on his horse farm in Georgia.


One Response to “Cannabis And Mental Illness”
  1. Psychiatric Resource Forum says:

    Marijuana and Mental Illness

    There seems to be a growing amount of research that is pointing directly at the use of marijuana in youth as a risk factor for developing a mental illness later on in life. While the exact mechanism is not fully

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