Medical research shows a clear link between marijuana use and mental illness.
By SAMUEL T. WILKINSON
Recent legislation has permitted the recreational use of marijuana in Colorado and Washington state. Those who support legalization often tout the lack of serious medical consequences associated with the drug. Most of us know people who used marijuana in high school or college and seem to have suffered no significant medical consequences. As the medical and scientific literature continues to accumulate, however, it is becoming clearer that the claim that marijuana is medically harmless is false.
There is a significant and consistent relationship between marijuana use and the development of schizophrenia and related disorders. Schizophrenia is considered by psychiatrists to be the most devastating of mental illnesses. Patients who suffer from it often experience auditory or visual hallucinations, severe social withdrawal and cognitive impairment. Many require frequent and prolonged hospitalization in psychiatric wards.
Schizophrenia affects almost three million Americans—more than six times the number of people with multiple sclerosis, two and a half times the number of people with Parkinson's disease, and more than twice the number of people with HIV/AIDS. Less than one-third of patients with schizophrenia can hold a steady job or live independently. A large portion (about one-third) of homeless people in the U.S. suffer from the disease.
Though they receive little attention in the legalization debate, the scientific studies showing an association between marijuana use and schizophrenia and other disorders are alarming. A 2004 article in the highly respected British Journal of Psychiatry reviewed four large studies, all of which showed a significant and consistent association between consumption of marijuana (mostly during teenage years or early 20s) and the later development of schizophrenia. The review concluded that marijuana is a "causal component," among others, in the development of schizophrenia and other psychotic disorders.
A 2007 study in the Lancet, a British medical journal, concludes that using marijuana increases the risk of young people developing a psychotic illness, such as schizophrenia. This risk is greatest—up to a 200% increase—among those who use marijuana heavily and who start using at a younger age.
Those not familiar with epidemiological causation may wonder how cannabis could "cause" schizophrenia if so many people who smoke marijuana or hashish don't develop the disease. As an example, medical researchers have known for several decades that smoking causes lung cancer, yet over 80% of smokers do not develop lung cancer.
As research accumulates, the emerging picture is that marijuana precipitates schizophrenia or related psychotic disorders in people whose brains are inherently vulnerable to psychosis. All of us who do not regularly experience hallucinations or delusions reside on what may be called a "cliff of sanity." Some of us, for reasons still unclear (thought possibly to be genetic), are closer to the edge of the cliff than others.
Marijuana may push everyone a few feet closer to that cliff. For those who were already close to the cliff, the drug pushes them over the edge into the chasm of insanity, hence precipitating the development of schizophrenia.
The association between schizophrenia and marijuana is not the only issue at play in the debate over marijuana legalization. If legalization is certain to decrease the power of drug lords in Mexico and other countries, then this is certainly a favorable outcome. However, if the trade-off is that more people suffer from schizophrenia—and thus more Americans are homeless and debilitated—then this must be recognized and discussed by the general public. This association between marijuana and serious and devastating psychotic disorders has been absent or under-recognized in the public debate.
Despite (or perhaps because of) the unwarranted stigma that surrounds their illness, individuals with schizophrenia are vulnerable and in need of advocacy. We owe it to them, and to society in general, to consider all the facts, risks and potential benefits before we embark on this drastic social experiment. If the end of Prohibition offers any historical precedent, once marijuana is legalized it will be all but impossible to undo.
Dr. Wilkinson is a resident physician in the Department of Psychiatry at The Yale School of Medicine. His opinions do not necessarily reflect those of The Yale School of Medicine or its Department of Psychiatry.
A version of this article appeared July 2, 2013, on page A13 in the U.S. edition of The Wall Street Journal, with the headline: Pot-Smoking And the Schizophrenia Connection.