Cannabis advocates are beginning to find more and more evidence to suggest that cannabis has medicinal properties and is safe for responsible recreational use. However, there is a growing body of research linking cannabis with a number of problematic outcomes including birth complications, cancer, and motor vehicle crashes.

In this age of increasing legalization of cannabis, advocates must contend with this research, and, when appropriate, concede any potential risks associated with marijuana use.

However, the emerging state of cannabis research works both ways: 

Advocates may not be able to argue that cannabis is completely harmless and is safe to use under any circumstances, but critics of cannabis must acknowledge the weaknesses present in many studies that support their position.

To this end, below we acknowledge the strengths of some of the more alarming research findings that have recently been published about cannabis. We also point out some weaknesses of these studies to consider.

What Does Weed do to Pregnant Women?

Findings: This 2016 meta-analysis (an analysis of the findings of multiple studies; in this case, twenty-four) reported that, compared to pregnant women that did not use cannabis, pregnant women who used cannabis during pregnancy were more likely to have anaemia (a lack of red blood cells in the blood), as well as neonates (babies) with lower birth weight and a greater likelihood of being placed in a hospital neonatal intensive care unit before being brought home.

Study Strengths: There is reason for concern regarding whether pregnant women should consume cannabis. Research in this area is still largely lacking and, of course, pregnant women should always be cautious with any substances that they put in their bodies.

Study Weaknesses: To the authors’ credit, they admit a major flaw in their study: 

“Many cannabis users are often tobacco or alcohol users; hence, determining a cannabis-only effect (excluding the presence of tobacco and alcohol) was currently not possible, as most studies did not exclude participants with polysubstance use.” 

Notably, this study did exclude women who were using other illicit drugs besides cannabis, but that is clearly not enough to pinpoint cannabis as the cause of the adverse outcomes that were analyzed. This is a critique that must be addressed in any research suggesting that there are harmful effects of marijuana.

Does Marijuana Cause Cancer?

Findings: This 2015 meta-analysis of three studies found a link between current, chronic, and frequent cannabis use and testicular germ cell tumors.

Study Strengths: The authors theorize that “. . . cannabis exposure – and subsequent stimulation of cannabinoid receptors – disrupts normal hormone regulation and testicular function, and that this disruption leads to carcinogenesis [cancer formation].” This is an important reminder that using cannabis to activate the human body’s endocannabinoid system is extremely complex and is not to be taken lightly. 

Study Weaknesses: All three of the studies that were analyzed asked participants if they have ever used cannabis. The men that answered “yes” to this question were found to be at greater risk for testicular cancer than those who answered “no.” However, we must consider the many potential differences between these two groups of men. For starters, those who have tried marijuana before are perhaps more likely to have used tobacco (see our response to the pregnancy study, above). Indeed, we cannot assume that the only difference between these two groups is whether or not they have used cannabis; the researchers failed to account for other potentially meaningful differences between these groups — therefore, we can’t say whether this finding reflects an actual real-world phenomenon.

Is it Safe to Drive High?

Study: Drivers that use cannabis are at an increased risk of being involved in motor vehicle crashes.

Findings: A 2012 meta-analysis of nine studies conducted across the U.S., Canada, New Zealand, Thailand, and parts of Western Europe found that drivers who use cannabis are more than twice as likely as other drivers to be in motor vehicle crashes.

Study Strengths: Using cannabis while driving is a risk that should be avoided — and this study backs up this claim quite well, though the studies could generally not determine if drivers were under the influence of marijuana at the time of the crash. Notably, the authors attempted to isolate the effect of cannabis on motor vehicle crashes by controlling for other factors, such as alcohol use.

Study Weaknesses: As we noted above, we cannot state that cannabis alone is responsible for the crashes recorded in these studies; other factors like risky driving behavior could have contributed as well. This study was also concerned with either self-reported cannabis use or blood/urine tests for THC, meaning that the many other cannabinoids commonly found in cannabis were not accounted for. Thus, we don’t know whether certain cannabis strains that have lower levels of THC are safer to use. 

Given the importance of this topic as more and more U.S. states legalize cannabis, we should also point out that a 2017 study analyzing the U.S. Fatality Analysis Reporting System reported this finding: 
“Three years after recreational marijuana legalization, changes in motor vehicle crash fatality rates for Washington and Colorado were not statistically different from those in similar states without recreational marijuana legalization.”

Is Weed Smoke Safe?

Study: Cannabis use might elevate the risk of lung cancer.

Findings: Heavy cannabis smokers were more than twice as likely to develop lung cancer over a 40-year follow-up period, even after accounting for tobacco use, alcohol use, respiratory conditions, and socioeconomic status.

Study Strengths: Any way you slice it, inhaling smoke is not very good for you; the authors point out that cannabis smoke and tobacco smoke contain many of the same carcinogens.

Study Weaknesses: Cannabis research generally makes no distinction between various consumption methods. Would these researchers have found this same result if they looked at people using vaporizers only? What about edibles only? Perhaps not.

Also, contradictory research argues that marijuana smoke and tobacco may not be quite so similarly harmful: "While chemically very similar, there are fundamental differences in the pharmacological properties between cannabis and tobacco smoke. Cannabis smoke contains cannabinoids whereas tobacco smoke contains nicotine."


As cannabis research continues to proliferate, advocates and critics alike must acknowledge the strengths and weaknesses of the studies that support or conflict with their positions. On one hand, advocates ought to at least acknowledge the possibility of evidence-based risks associated with cannabis. Meanwhile, critics should be aware of the serious limitations that many cannabis studies have.

If both parties are willing to meet each other halfway in this manner, discourse around marijuana use will continue to improve for the benefit of everyone living during this unprecedented time of legalization in cannabis history.