One of the common questions relevant to the debate on medical marijuana use and legalization is, “How does cannabis affect mental illness?” To understand this question fully, it is necessary to understand some basics about the field of research, to assess the available evidence surrounding this question, and to consider the possible application of marijuana in treating symptoms of mental health conditions.
Why Do People Think Cannabis Causes Mental Illness?
Many studies have found that people who consume cannabis tend to be more likely to have various types of mental health issues, such as depression, anxiety, and psychotic disorders. As a result of these studies, people have incorrectly inferred that marijuana use causes mental illness.
However, this assumption stems from a basic misunderstanding of research results; namely, the fact that correlation does not imply causation. You may have heard this before. So what does it mean, and why does it matter?
Correlation vs. Cause-Effect Relationships
Correlation is a link or association that is found between two variables. There can be a negative correlation between variables, meaning that when one variable increases in magnitude/frequency, the other decreases in magnitude/frequency. For example, the more you exercise, the less likely you are to gain weight.
You can also have a positive correlation between variables, meaning that when one variable increases in magnitude/frequency, the other does so as well. For example, the more often you eat high-salt foods, the more likely you are to develop certain health problems such as high blood pressure.
However, a correlational relationship is not equivalent to a cause-effect relationship. That is to say, just because there is a correlation between two variables, it does not mean that one caused the other. For instance, one of the most frequently cited examples of when correlation does not imply causation goes as follows: In the summertime, ice cream sales increase, and murders also increase. So that means that when people buy more ice cream, this causes people to have an increased desire to go out and kill people, right? Wrong!
There could be a third, unseen factor causing both an increase in ice cream sales and murders— heat! As the heat rises, people like to eat cold foods/desserts (e.g. ice cream). Heat may also cause people to become more agitated, angry, and frustrated, and they are more frequently outdoors in close proximity with other individuals. All of this together can unfortunately result in an increased murder rate.
Correlation relationships can be helpful in assessing, understanding data, and can help develop research that will assess the possibility of a cause-effect relationship between variables. But a causative link between two variables cannot be drawn from the mere existence of a correlation between them.
This misunderstanding of correlation relationships is also partially responsible for the widely-held belief in the “gateway theory”, which incorrectly asserts that cannabis use leads to a desire for more dangerous drugs, such as heroin or cocaine.
The Importance Of Understanding Selection Bias
The frequent conclusion that cannabis causes mental disorders, drawn from studies showing a link between marijuana use and mental illness, may be a directional misinterpretation. That is, it is very possible that users are more likely to utilize medical marijuana because they have a mental health disorder, and not that it actually leads to the development of mental health disorders. This can lead to a “selection bias” in studies.
According to one source, selection bias is defined by “systematic differences between comparison groups in prognosis or responsiveness to treatment”. Most studies conducted evaluating the impact of marijuana on mental illness have used recreational consumers as participants, and therefore begin the study with inherent selection bias.
Some people with mental disorders such as depression, anxiety, or psychotic mental disorders may choose to self-medicate with marijuana, individuals with schizophrenia may be more likely to use marijuana possibly due to their genetic makeup, and certain psychotic illnesses which already exist may be exacerbated by marijuana use. Therefore, the results of many studies which find links between marijuana use and mental health disorders do not provide sufficient evidence that marijuana causesmental illness; the group that consumes cannabis is likely inherently different than their counterparts from the outset of the study.
If there are differences between the two groups before the study has even begun, it is impossible to assess whether differing results for the two groups (i.e. mental illness or no mental illness) is due to cannabis use or due to some other factor that made the two groups different in the first place. Therefore, these studies using self-selected recreational consumers as participants do not provide valid evidence that cannabis causes mental illness.
Additionally, a recent retrospective study conducted at Harvard shows that marijuana use does not cause schizophrenia specifically.