Does Marijuana Cause Mental Illness?

The cannabis plant, which yields marijuana, has been cultivated for centuries for the production of hemp fiber and for its medicinal and psychoactive properties. Surveys indicate that marijuana is the most commonly used illegal drug in the United States. Its use peaked in the 1970s when about 60 percent of high school seniors reported having used it and 11 percent reported daily use. The use of marijuana declined in the 1990s and rose again in around 2002, with about 48 percent of high school seniors reporting having used it. Today, the association between marijuana and mental illness has caused a renewed interest in its use.

An association between marijuana use and mental illness, particularly psychosis, has long been recognized. Recently, with a better understanding of brain function and pharmacology, researchers have renewed their interest in this subject. Although present statistics suggest that marijuana accounts for 10 percent of all cases of psychosis, epidemiological evidence suggests that the number could be higher. Patients with psychosis are at higher risk for cardiovascular and metabolic diseases compared to the general population.

Researchers at Yale University School of Medicine report that cannabinoids, which are the active ingredient in marijuana, can induce acute transient mental illnesses, particularly psychotic symptoms, including paranoia and acute psychosis in some individuals. As to why some people experience this and others don't remains unclear. The Yale researchers also note that in individuals with an established psychotic disorder, cannabinoids can exacerbate or worsen psychotic symptoms. These exacerbations can persist long past the period of intoxication. Available evidence suggests a causal relationship between marijuana, psychosis and the development of schizophrenia. However, because most people who use marijuana don't develop these disorders, the Yale researchers theorize that cannabinoids interact with another component, perhaps genetic risk, that predisposes them to psychosis. Dysfunction of the CB1 receptor has been proposed as a possible risk factor and is being further investigated.

Worldwide, researchers are studying the effects of marijuana on mental health. In 2008, French researchers published a study in which they evaluated 32 cannabis abusers along with 30 healthy controls using self-reports. Their study showed that cannabis abusers were more likely to have mood and anxiety disorders and to experience high levels of psychological distress, trait anxiety, physical anhedonia, and sensation seeking than controls. A similar study from the Netherlands showed that acute cannabis use can cause memory impairment persisting for months after its use. Cannabis, however, showed no changes in brain anatomy based on MRI reports.

There are at least six distinct cannabinoids present in marijuana. The two most important active ingredients in marijuana are the cannbinoids Delta 9-tetrahydrocannabinol (delta9-THC) and cannabidiol (CBD). The amounts of these two components present in marijuana can vary and they both cause different effects. In recent years plant engineering has maximized levels of delta9-THC, thereby increasing the plant's potency. Delta9-THC increases anxiety and levels of intoxication, sedation and psychotic symptoms, whereas CBD reduces anxiety and acts on the amygdala and anterior and posterior cingulate cortexes, affecting the limbic and paralimbic regions.

The pharmacological effects of delta9-THC vary with the dose, route of administration, experience of the user, vulnerability to psychoactive effects and setting of use.

Resources

DC D'Souza Cannabinoids and Psychosis, Int Rev Neuorbiol 2007l 78: 289-326.
P Fusar-Poli, J Crippa, et al, Distinct effects of delta9-tetrahydrocannabinol and cannabidiol on neural activation during emotional processing, Arch Gen Psychiatry, January 2009; 66(1): 95-105.