With marijuana now permitted in some form in 23 U.S. states, the usual flow of pot from south to north has slowed and, to a growing degree, reversed. This was never imagined as a benefit of Nafta. Now, the expanding U.S. pot industry is transforming the drug distribution patterns of the notorious cartels—forcing them to deal more exclusively in heroin, for example—and leading to both cultural and economic change in Mexico’s own consumption of marijuana. Two opportunities may arise: a business boom for legal pot producers in the U.S. and the chance to concentrate the drug war on far more deadly substances.

The effects are being felt in Sinaloa, long the heart of Mexican pot production. Farmers there are ripping out marijuana planted on hillsides. “In our town, it’s dropped because it’s no longer a profitable business,” says Mario Valenzuela, mayor of Badiraguato, the hometown of infamous drug lord Joaquín “El Chapo” Guzmán, who was arrested in 2014. Over the last two years, participation in a program that subsidizes farmers who plant crops like tomatoes or green beans instead of marijuana has increased 30 percent, Valenzuela says. He attributes the increase to the surge in U.S. production since legalization.

In the past, only a sixth of cannabis consumed in the U.S. was grown within the 50 states; today that’s up to at least one-third, according to the United Nations. Pot from Colorado and California has started to displace the low-grade stuff that’s long flowed in by truck, tunnel, human mule, and boat from Mexico. Marijuana seizures by U.S. Customs and Border Protection at California border crossings totaled 132,075 pounds in fiscal 2014, half of the amount five years earlier.