Back in 1936, there was a movie released called “Reefer Madness” that became a cult classic. When I first saw it in the 1970s, it was considered propaganda about the evils of smoking marijuana. The most prominent of those evils was the danger to your mental health. The message was clear; marijuana consumption was likely to make you crazy.

However, the whole premise of the film was made fun of by the counterculture of that time. Even though marijuana was strictly illegal then, it was widely available on college campuses and elsewhere and was viewed as a recreational drug, likely less harmful than alcohol.

After I began practicing medicine near my childhood home, I noticed that some of the people I knew while growing up had developed schizophrenia and other life-altering mental derangements. Since some of them had been “stoners” when they were growing up, I wondered whether they had previously been trying to self-medicate their mental and psychological imbalances or whether street drugs might have played a role in causing their long-term problems.

This concern returned to my mind recently as I drove to Detroit recently and saw multiple billboards advertising recreational cannabis for sale. They seemed to have sprung up everywhere after Michigan recently legalized recreational as well as medicinal marijuana. (Indiana has not even approved medicinal marijuana thus far.)

Then, I read a summary of a Danish study published in JAMA Psychiatry about a possible causal association between cannabis use and schizophrenia.

The researchers pointed to a clear increase in the proportion of schizophrenia cases linked to cannabis use disorder over the past 25 years.

However, it is nearly impossible to use epidemiological studies to actually prove causation. What they state is that all the numbers behave exactly in the way that would be expected under the theory of causation.

The findings are based on Danish national health registry data. The study sample included all people in Denmark born before Dec. 31, 2000, who were aged 16 years or older at some point from Jan. 1, 1972, to Dec. 31, 2016.

Despite some fluctuation, there was a general increase in the population-attributable risk fraction for cannabis use disorder regarding schizophrenia over time. The fraction increased from about 2% in 1995 to about 4% in 2000 and has hovered from 6% to 8% since 2010.

The findings are particularly important with the increasing legalization of cannabis for both medicinal and recreational uses seeming to lead to an increase in the perception of cannabis as relatively harmless and possibly in the uptake of cannabis use, especially among youth.

Perhaps the evidence is not strong enough to change the current scientific view of cannabis use as a “component cause” that interacts with other risk factors to cause schizophrenia. It may not be either necessary or sufficient to cause schizophrenia on its own.

Certainly, psychosis is not the only outcome of interest in terms of cannabis use. The intake of marijuana by smoking or swallowing it has a significant effect on the ability to think clearly.

Now that cannabis is available legally within a short distance from our homes, it is important that we realize that it should not be considered harmless.

The idea that cannabis use and schizophrenia may be related needs a great deal of further study, but it might be real to some extent.

In the meantime, I am not willing to go back to the “Reefer Madness” days when marijuana was thought by some to be an evil weed. But I do think it is very important to set parameters regarding its use, similar to those established for alcohol use in order to avoid putting more impaired drivers on the road and in other dangerous situations.

One does not need to go far to meet someone who has been injured or has lost a loved one because of a drunk driver. We do not need to create a situation where similar stories will be common because of drivers impaired by cannabis use.

Dr. Terry Gaff is a physician in northeast Indiana. Contact him at or on Facebook. To read past columns and to post comments go to

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