President Trump says his border wall will keep drug cartels from flooding American streets with heroin, but that plan is doomed to fail according to the DEA.

While addressing America's opioid epidemic last month, Trump promised that the massive wall he intends to build along the U.S.-Mexico border would stem the flow of heroin into America. "An astonishing 90 percent of the heroin comes from south of the border, where we will be building a wall which will greatly help in this problem," he vowed.

Trump is right in saying most of America's illicit heroin supply originates from Mexico, but trying to keep drugs out with a giant wall will work about as well as carrying water in a sieve. The majority of Mexican Transnational Criminal Organizations (TCOs) don't smuggle drugs into the country by crossing the border illegally. They go through official ports of entry, according to the DEA.

"Mexican TCOs transport the majority of illicit drugs into the United States across the SWB [Southwest Border] using a wide array of smuggling techniques," the DEA wrote in their 2017 edition of the National Drug Threat Assessment report. "The most common method employed by these TCOs involves transporting illicit drugs through U.S. ports of entry (POEs) in passenger vehicles with concealed compartments or commingled with legitimate goods on tractor trailers."

After sneaking their illicit cargo across the border, those Mexican TCO's distribute their drugs via mules with bus tickets. "Heroin shipments are sent via couriers on passenger buses," the DEA wrote. 

That means Trump would have better luck curbing the supply of heroin imported from Mexico by banning Greyhound than by building a border wall.

So there's not much the Trump administration can do to stop the problem with supply, but he could curb demand by researching medical marijuana as an alternative to the prescription pills fuelling America's opioid epidemic. Finding that much-needed alternative would stop people from turning to heroin in the first place and put Mexican TCOs out of business.