Does it really, though?
New research published in the journal Health Economics suggests that the argument is overstated. Darin F. Ullman, an economist who recently received his PhD from the University of Wisconsin at Milwaukee, wanted to know what effect, if any, the enactment of medical marijuana laws has had on employee absentee rates.
A fair amount of research has been done on the aggregate impact of illicit marijuana use on workplace productivity. Generally speaking, the most recent research — gathered and summarized in this 2014 paper — indicates that most marijuana use has little effect on workplace productivity, although chronic or heavy pot use can be a problem.
On net, the evidence is mixed. "It is simply uncertain as to whether there are negative labor market consequences of drug use in general, and cannabis use in particular," the 2014 paper concludes.
But there hasn't been a lot of research into the impact of licit marijuana use — particularly medical marijuana use — on the workplace. So Ullman decided to look into what happened to employee sick-day use in states that legalized medical marijuana, according to the Bureau of Labor Statistics' Current Population Survey (CPS).
On the one hand, you might expect broader access to marijuana to result in more workers calling in sick, because they're too stoned to work or because they just don't feel like showing up on a given day. On the other hand, if medical pot is successfully treating conditions that would otherwise render somebody unable to work, you might expect sick days to decrease.
So Ullman examined before-and-after sick-day data from 24 states that had medical marijuana laws at the time of his study. On average, he found that "respondents were 8% less likely to report being absent from work due to health issues after medical marijuana laws" were passed. The CPS numbers also suggest that states with fewer restrictions on the use of medical marijuana, such as on the number of conditions it could be recommended for, had more of a decrease in sick-day use than states with stricter regulations.
Now, if you read much about this type of research, you're probably expecting a Big Important Caveat to appear here, and you're right: Ullman's study can say that sick-day use decreased after the passage of medical marijuana laws. But it can't say medical marijuana caused that decrease. There are any number of factors that could have accounted for a drop in absenteeism in the states Ullman studied — better access to health care, better workplace wellness programs, improved employee health overall, etc. The decline in absenteeism could be driven more by any of those factors than by whatever happened to the state's marijuana laws.
However, Ullman notes that the effect of the laws was stronger for middle-aged workers and for males, the groups most likely to hold medical marijuana cards. That, combined with the stronger effect of the laws in the more lax states, does suggest that the laws themselves could be a driver of the reduced absenteeism seen in the data.
Ullman notes that there are any number of plausible reasons this could be the case. If self-treating with medical marijuana lets "individuals experience relief from disabling symptoms, absence from work could decline." Beyond that, other studies have shown that alcohol consumption declines after the passage of medical marijuana laws. Heavy drinking is a big driver of absenteeism, so if medical marijuana cuts back on boozing, it would have the net effect of reducing absenteeism, as well.
"The results of this paper therefore suggest that [medical marijuana laws] would decrease costs for employers as it has reduced self-reported absence from work due to illness/medical issues," Ullman concludes.