Twenty states and the District of Columbia have legalized medical marijuana under the logic of helping patients manage chronic pain. This has also made it possible for many recreational users to score weed legally, and that may be a good thing for everyone - non-users and disapproving parents included. By substituting for alcohol, recreational marijuana use may reduce the prevalence of drunk driving and save lives.

Three researchers published evidence of this effect in a recent paper entitled “Medical Marijuana Laws, Trafﬁc Fatalities, and Alcohol Consumption.” The paper investigates the impact of medical marijuana laws. But given that most prescriptions are for self-reported pain symptoms, government monitoring is costly and difficult, and dispensaries and home cultivation make it easy to share with recreational users, they do not just look at medicinal use of pot.

(An earlier working paper is available here and the final paywalled article can be accessed here.)

The authors look at 3 states that legalized medicinal marijuana in the mid 2000s: Montana, Rhode Island, and Vermont. Using data from the National Survey on Drug Use and Health, they find that marijuana use increased in Montana - where 3% of the population is registered under medicinal marijuana laws - by 1.7%. That’s 0.9% more than in neighboring states that have not legalized marijuana. Rhode Island saw a more modest increase and Vermont, which did not allow dispensaries until 2011 and has under 500 registered patients, saw no significant increase.

Traffic fatalities are the leading cause of death among Americans ages 5-34, with alcohol impaired driving responsible for ⅓ of roughly 10,000 annual deaths. Both weed and alcohol slow reaction time and otherwise mess with driving abilities, but marijuana seems to be much less impairing:

Drivers under the influence of marijuana reduce their velocity, avoid risky maneuvers, and increase their “following distances,” suggesting compensatory behavior. In addition, there appears to be an important learning-by-doing component to driving under the influence of marijuana: experienced users show substantially less functional impairment than infrequent users.

While doctors would not recommend testing these findings, the authors note that “neither simulator nor driving-course studies” have consistently shown that marijuana use increases the risk of accidents. Drunk drivers, in contrast, underestimate their inebtriation, take more risks, drive faster, and clearly are at a higher risk of collision. So if people smoke weed as a substitute for drinking, then we should expect to see safer roads - or at least less accidents.

The study tests this hypothesis by looking at reports of fatal injuries from traffic accidents collected by the National Highway Traffic Safety Administration. In the 3 states of interest, they find that legalization decreased the number of fatal crashes per 100,000 drivers by 12% in accidents involving a positive blood alcohol content. Fatal crashes involving high blood alcohol contents decreased 14%.

Some additional findings bolster the case that increases in the number of people reaching for a pipe rather than drinking and ultimately driving explain these results. The decrease became more significant each year after legalization and was more pronounced during the weekend and at night (when people tend to drink and smoke) than during weekdays. The effect was also stonger among men, who use marijuana more frequently than women.

Alarmingly for owners of liquor licenses, a national survey investigating alcohol use also revealed that legalization decreased the mean number of drinks consumed per month by 1.51 among men and 0.65 among women. Analysis of beer sales data, when controlled for other variables, showed that medicinal marijuana is “associated with a 5.3% reduction in beer sales.”

This all suggests that when legalization increases access to marijuana, people respond by drinking less beer and smoking more, which in turn reduces the number of people who die in drunk driving accidents each year. If we (very unscientifically) extrapolate the reduction in alcohol related traffic fatalities to the entire country, a national medical marijuana law would save roughly 870 people from dying in car crashes every year.

The authors remain agnostic about whether driving high is safer than driving drunk and note that driving under the influence of alcohol and marijuana may be even more dangerous than just driving drunk. A current working paper on the topic also emphasizes that this study looks at only 3 states and may not account for the different implementations of medicinal marijuana laws. But if the study’s findings that people substitute weed for beer are correct, then legalization also has a positive public health benefit simply by reducing the incidence of drinking.

It’s not immediately obvious that pot is less harmful than alcohol. After all, one is illegal and the other is not. In 1970, the Drug Enforcement Agency gave marijuana its most dangerous classification as a schedule 1 drug. This defines weed as having “no currently accepted medical use and a high potential for abuse… with potentially severe psychological or physical dependence.”

This technically means that marijuana is as dangerous as heroin and more harmful than cocaine. If that sounds wrong to you, well, it is.

Research on marijuana tends to begin by summarizing disagreement over its merits. Critics decry weed by saying it is “addictive, serves as a gateway drug, has little medicinal value, and leads to criminal activity.” Advocates note the many demonstrations of its medical benefits and cast doubt on its risk factor and association with criminal activity.

But the disagreement among researchers reflects a self-fulfilling prophecy of investigating the effects of a drug whose illegal status branded it as dangerous and restrained researchers ability to prove otherwise.

In a recent high-profile article about medical marijuana, Dr. Sanjay Gupta, CNN’s chief medical correspondent, argues that pot’s schedule 1 designation skews medical research. Sourcing weed is extremely difficult given its legal status and studies require additional approval from the National Institute on Drug Abuse, “an organization that has a core mission of studying drug abuse, as opposed to benefit.”

The Assistant Secretary of Health announced marijuana’s classification as a schedule 1 drug in 1970 with the following words:

"Since there is still a considerable void in our knowledge of the plant and effects of the active drug contained in it, our recommendation is that marijuana be retained within schedule 1 at least until the completion of certain studies now underway to resolve the issue."

However, Gupta argues that not only were those “certain studies” never completed, but the announcement ignored years of medical research demonstrating its benefits. After all, marijuana had been a standard pain treatment in pharmacology.

The skewed marijuana research performed in recent years convinced many healthcare professionals that the medical case for weed was weak. That includes Dr. Gupta, who previously argued against legalization but apologizes for that stance in his article.

His more rigorous look at past studies and current research from smaller labs outside the country led him to join the growing consensus that marijuana is a drug with powerful medical benefits and low, manageable risks.

From a recreational perspective, this consensus reflects most people’s intuition that pot is safer than its legal counterparts tobacco and alcohol. A night of too much weed results in bad poetry, a bad case of the munchies, or a bad trip - no one has ever died from a marijuana overdose. In contrast, paramedics regularly rush teens to the hospital and pump their stomachs in an attempt to save their life and drunk driving is a leading cause of death in America. The dependency rate of marijuana is around 9% compared to 30% for tobacco, and even then “marijuana [does] not lead to significant addiction in the medical sense of the word.” Compare that to the scourge of alcoholism. Scant evidence exists that weed does act as a gateway drug to heroin or cocaine use.

Marijuana is not harmless. It's unhealthy for the lungs, has some association with depression, and has measurable effects on memory in long term users. Doctors also worry about the consequences for teenage smokers, and focus current research and commentary on comparing medical use of marijuana to other medical drugs. But overall, "the risks of marijuana use, which have been thoroughly researched, are real but generally small."

People regularly choose to modify their mental state with substances ranging from caffeine, tobacco, marijuana, and alcohol to drugs like heroin and cocaine. People treat some of these as substitutes, but not others. In this context, criminalizing pot - one of the least risky options out there - leads people to choose more dangerous substances. From that perspective, it should come as no surprise that decriminalizing marijuana will have positive effects.

This post was written by Alex Mayyasi. Follow him on Twitter here or Google Plus. To get occasional notifications when we write blog posts, sign up for our email list.