As of May 2013, 20 states and the District of Columbia have passed medical marijuana laws with several more states considering similar legislation.  Although the details of the law vary from state to state, the primary legal implication is that state-level penalties for medical marijuana use, possession, and cultivation are removed.  Moreover, 15 states have decriminalized marijuana use, which removes criminal penalties for first-time possession of a small amount of marijuana for personal consumption.  To our knowledge, no study has explored the effects of these law changes on body weight.

A recent paper by Anderson, Rees, and Hansen (2013) finds that medical marijuana laws are associated with a decline in the price of high quality marijuana as well as a decline in binge drinking among younger males.  In addition, Pacula et al. (2003) find that marijuana consumption is quite sensitive to state criminal penalties imposed.  Our paper extends this line of research to explore the effect of state marijuana laws on diet, exercise, and, ultimately, body mass index (BMI).

The effect of medical marijuana laws (and marijuana decriminalization laws) on weight is theoretically ambiguous.   If individuals substitute marijuana for higher calorie alternatives—such as alcoholic beverages, high-fat foods, or larger meals—then reducing the price of marijuana may cause body weight to fall.  In addition, avoiding binge-drinking-induced hangovers may increase energy and exercise.  Alternatively, marijuana consumption may induce lethargy or increase one’s appetite for easy-to-access or ready-made, high calorie foods (“the munchies”), which may increase body weight.  Finally, if marijuana and tobacco are substitutes, and cigarette consumption serves as an appetite suppressant, then this could be another mechanism through which medical marijuana (or marijuana decriminalization) laws increase body weight.

While a number of descriptive studies find evidence of a positive relationship between marijuana use and body weight, most of these studies have treated marijuana use as exogenously determined.  In fact, weight is a health outcome produced using inputs, some of which are exogenous (e.g. genetic endowment), but many of which are endogenous, such as marijuana use.  There are a myriad of difficult-to measure family background and individual traits—such as home environment, personality, and personal discount rates—that may be associated with both marijuana consumption and body weight.  French (2012), in fact, finds that the estimated association between marijuana use and BMI is quite sensitive to the inclusion of controls for observable measures of socioeconomic status and health behaviors.

The current study will be the first in the literature to estimate the effect of medical marijuana laws—which provide arguably exogenous variation in the price of marijuana—on BMI, exercise, and health BMI.  Specifically, we use data drawn from the Behavioral Risk Factor Surveillance Survey from 1990 to 2011 and a difference-in-difference approach to estimate the health effects of marijuana law changes.  Preliminary estimates suggest that medical marijuana laws reduce BMI among individuals over age 40, but have little effect on those in their 20s.  These findings point to medical and non-medical uses of marijuana induced by these law changes.