While more and more states are legalizing marijuana by popular vote, some politicians still claim that marijuana is a gateway drug. Various medical studies have proven that claim to be false. 

 The gateway theory claims that people who are addicted to heroin, crack cocaine, crystal meth or bath salts often used marijuana prior to graduating to harder drugs, so some politicians, who typically have no medical training whatsoever, assume that there is some sort of causal mechanism that desensitizes drug users, and pushes them down a slippery slope towards harder and more powerful drugs.

However, medical research shows that there is no evidence for that. In fact the evidence proves the opposite. There’s a wealth of research showing the flaws in the gateway theory. Unfortunately, the common thread among these studies is that much of them come from outside the U.S. or from grassroots organizations within the U.S. that are promoting marijuana legalization.

A University of California, Berkeley, study recently published in the journal Addiction examined Amsterdam's 30-year history with freely available marijuana. In the article, Robert J. MacCoun, a professor at UC Berkeley’s Goldman School of Public Policy and the UC Berkeley School of Law, said that the Dutch seem to have reduced the link between marijuana users and hard drug users and sellers by separating the markets for each via the cannabis coffee shop system.

Dutch citizens use cannabis at more modest rates than many of their European neighbors. The Dutch continuation rate for using marijuana from a causal experimentation in youth to regular usage in adulthood (ages 15-34) is fairly modest by international standards. Past-year cannabis use among Dutch 15-to-24-year-olds dropped from 14.3 to 11.4 percent between 1997 and 2005.

Dutch youth resemble American youth in that they are likely to start using marijuana before they turn 13, even though the age limit for legal purchases in the Netherlands was 16 until 2005, when it was increased to age 18.

For his study, MacCoun compared available data on the prevalence and patterns in the Netherlands of marijuana use, treatment for marijuana addiction, and the sanctioning and prices for marijuana, with similar data for Europe and the United States.

When analyzing what acts as a "gateway" to hard drug use, there are a number of factors at play. None involve marijuana. Poverty and poor social environment is a gateway to drugs, according to much research. Association with people who use hard drugs is a better predictor of harder drug use. Certain mental illnesses, such as antisocial personality and bipolar disorder, are found to predispose some people to use drugs. Other research notes that criminalization and prohibition are real gateways to harder drugs.

So why do some American politicians still oppose the legalization of marijuana? A closer look reveals the historical roots, and vested interests, behind the marijuana prohibition.

As Nathan Greenslit explained in an Atlantic article, U.S. drug policy began with racist fear-mongering by Federal Bureau of Narcotics director Harry Anslinger in 1937.

The Nixon administration strengthened drug control with the creation of the Drug Enforcement Agency, which classified marijuana as a Schedule 1 drug, against the advice of the National Commission on Marijuana and Drug Abuse.

Because marijuana is still officially classified in the U.S. as a Schedule I drug with no medical value, carefully controlled research using marijuana must receive approval from several federal departments. On the rare occasions that researchers do get approval, local politics can thwart the study.

Meanwhile, in the United States, Drug Rehab has become big business. Addiction researchers and addiction treatment professionals are heavily invested in the poorly supported claim that marijuana is a gateway to hard drugs. For decades, American scientists who study addiction have received millions of dollars in government and pharmaceutical funding to perpetuate the gateway hypothesis. Many would lose their respected reputations (or continued funding) if a gateway mechanism is not a legitimate research goal.

Those who work in the vast addiction treatment profession are especially invested in keeping the gateway theory believable, since the majority of their treatment patients are marijuana users. Their jobs depend on a belief in addiction as a disease and on marijuana being an addictive drug.

Fears of decriminalization resulting in increased use among youth haven’t been supported by research from countries where drugs were decriminalized.  A 2004 study compared Amsterdam, where marijuana was decriminalized, to San Francisco, where cannabis was, at the time, still criminalized. The authors found that criminalization of marijuana didn’t reduce use, while decriminalization didn’t increase use. Nor has this trend been noted in studies of U.S. states that legalized marijuana for medical or recreational purposes. For example, in an article published in the American Academy of Pediatrics, the authors found no evidence that young people had increased marijuana use in states that had legalized medical or recreational marijuana.

Studies consistently find that the traumatic experience of being arrested and incarcerated for marijuana possession is the most harmful aspect of marijuana among young people. Arrest for possession can result in devastating, and often permanent, legal and social problems, especially for minority youth and low-income families.

According to studies by the ACLU, nearly half of all drug arrests were for marijuana possession, and the majority of those arrested were African-American. In some states, African-Americans were more than eight times more likely to be arrested for marijuana than whites.

Apart from racism, the largest factor in the marijuana debate is money. Cigarettes and alcohol have powerful lobbies, who give money to politicians to make sure they remain legal, no matter how addicting or harmful they are. These lobbies fear that if more people used marijuana, they'd consume less cigarettes and alcohol, thereby cutting into the profits of those lobbies.

And then there's the private prison lobby. More and more prisons in America are privately-run for-profit business enterprises. The more prisoners there are, the more money these prison corporations make. They lobby politicians to make laws as strict as possible, and to issue more and longer prison sentences, not to maintain law and order, but to increase profits. Legalizing marijuana would mean less people are sent to prison, and that would mean these prison corporations would make a lot less money.