The introduction of medical marijuana laws has led to a sharp reduction in violent crime in US states that border Mexico, according to new research.

According to the study, Is Legal Pot Crippling Mexican Drug Trafficking Organizations? The Effect of Medical Marijuana Laws on US Crime, when a state on the Mexican border legalised medical use of the drug, violent crime fell by 13% on average. Most of the marijuana consumed in the US originates in Mexico, where seven major cartels control the illicit drug trade.

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“These laws allow local farmers to grow marijuana that can then be sold to dispensaries where it is sold legally,” said the economist Evelina Gavrilova, one of the study’s authors. “These growers are in direct competition with Mexican drug cartels that are smuggling the marijuana into the US. As a result, the cartels get much less business.”

The knock-on effect is a reduction in levels of drug-related violence. “The cartels are in competition with one another,” Gavrilova explained. “They compete for territory, but it’s also easy to steal product from the other cartels and sell it themselves, so they fight for the product. They also have to defend their territory and ensure there are no bystanders, no witnesses to the activities of the cartel.

“Whenever there is a medical marijuana law we observe that crime at the border decreases because suddenly there is a lot less smuggling and a lot less violence associated with that.”

While the Mexican cartels smuggle other drugs such as cocaine, heroine and metamphetamine across the border, the market for marijuana is the largest drug market in the US and the one from which the cartels can make the fattest profit. It costs around $75 to produce a pound of marijuana in Mexico, which can then be sold on for $6,000 depending on the quality.

Gavrilova, along with fellow researchers Takuma Kamada and Floris Zoutman, studied data from the FBI’s uniform crime reports and supplementary homicide records covering 1994 to 2012. They found that among the border states the effect of the change in law was largest in California, where there was a reduction of 15% in violent crime, and weakest in Arizona, where there was a fall of 7%. The crimes most strongly affected were robbery, which fell by 19%, and murder, which dropped by 10%. Homicides specifically related to the drug trade fell by an astonishing 41%.

The authors claim their study provides new insights into methods to reduce violent crime related to drug trafficking. But its publication comes as the US attorney general, Jeff Sessions, is rescinding the Obama-era policy that ushered in the medical marijuana laws.

“When the effect on crime is so significant, it’s obviously better to regulate marijuana and allow people to pay taxes on it rather than make it illegal,” Gavrilova said. “For me it’s a no brainer that it should be legal and should be regulated, and the proceeds go to the Treasury.”

More than 20 states across the US have implemented medical marijuana laws so far. In those that have, there is now one marijuana dispensary for every six regular pharmacies.

The study suggests that the full legalisation of marijuana in Colorado and Washington will have an even stronger impact on the drug trade as large-scale marijuana production facilities are erected in these states, further threatening the position of drug cartels.

Anecdotal evidence suggests the cartels are now trying to get into the legal marijuana business in California by opening their own farms. Others are turning to human trafficking and kidnapping to shore up their falling profits. There are also reports that some cartels are switching to new forms of drug cultivation by growing poppies in Mexico to produce their own heroin rather than importing it from Afghanistan.