Recreational marijuana use is now legal in four states and "medical" marijuana in 23 states. Research on legalization policies has focused largely on direct impact - how they impact marijuana access and use. What is little discussed is that marijuana increases alcohol use.

Alcohol is the world's  most popular drug, the majority of adults in the U.S. imbibe to varying degrees and drinking accounts for almost one-third of driving fatalities annually. If you like pretend money estimates, it is claimed that alcohol use cost $223.5 billion in 2006 alone.

But people who willingly throw out pretend money estimates - dollars that maybe would have been virtually saved if alcohol didn't exist - suddenly understand skepticism and critical thinking when it comes to how much marijuana adds to alcohol use. Or takes away from it. If alcohol was a substitute for marijuana, and that is now gone, those billions should go down. But statistics instead show people suddenly do both. And those who use both substances simultaneously are twice as likely to drive drunk and face social troubles such as drunken brawls and relationship problems. Lead author Katarína Guttmannová, a researcher in the University of Washington Social Development Research Group, and colleagues conjecture that marijuana and alcohol both provide users with similar "reward and sedation" effects, which could prompt users to substitute one for the other. But blood levels of THC, the chemical responsible for most of marijuana's pleasurable psychological effects, increase with simultaneous alcohol use -- so the quest for a better high might lead people to use both substances.

The researchers reviewed more than 750 studies on marijuana and alcohol use and focused on 15 that specifically addressed the links between marijuana policies and drinking. They looked at how decriminalized marijuana, medical marijuana and recreational marijuana impacted alcohol use.

The findings of those studies fluctuated widely, depending on the demographic and the type and frequency of alcohol and marijuana use. One study, for example, found that states where marijuana is decriminalized had more emergency room visits related to marijuana and fewer visits linked to alcohol and other drugs. Some studies found that high school seniors in states where pot was decriminalized tended to drink less, while other research found that college students who used pot also drank more.

Findings around medical marijuana also varied, mostly because the reasons to get a card for it, "pain", are subjective. One study reported that states with medical marijuana dispensaries had higher rates of both marijuana and alcohol use, as well as higher admissions into alcohol treatment facilities. But while states with medical marijuana had fewer alcohol-related fatalities overall, those with dispensaries saw more of those deaths. 

Other research found that while legalized medical marijuana wasn't associated with any increases in underage drinking, it was linked with more binge drinking and simultaneous use of pot and alcohol among adults.