Trump promised to end the opioid painkiller and heroin epidemic. A wall won’t do that.

President Donald Trump said during his joint address to Congress on Tuesday that he will build “a great, great wall along our southern border” — in part to stop drugs from pouring in at an “unprecedented rate.” “Our terrible drug epidemic will slow down and ultimately stop,” Trump promised early on in his speech.

A solution is desperately needed. In 2015, mostly due to the opioid painkiller and heroin epidemic, more than 52,000 deaths were linked to drug overdoses — the highest single year of drug overdoses in US history.

There’s just one problem with Trump’s idea: The evidence suggests a wall will do little to nothing to stop the flow of drugs into America.

A 2013 report by journalist Reed Karaim summarized the expert consensus, following the previous few years of border security buildup:

Most border security analysts say there is little evidence the buildup has significantly reduced the availability of illegal narcotics in the United States. The U.S. Drug Enforcement Administration (DEA) has cited reduced use of some drugs, especially cocaine, as proof the buildup is working. But other drugs have grown in popularity, and smugglers have proved adept at shifting their methods and locations in response to interdiction efforts.

Peter Andreas, a political science professor at Brown University and author of Smuggler Nation: How Illicit Trade Made America, concurred with that conclusion. As he described it, no realistic amount of border security — including a wall — could actually stop the flow of drugs into the US, because drugs are simply far too profitable and compact (and therefore easy to smuggle) for drug traffickers to give them up.

“It’s very simple business economics that you’re not going to stop a commodity like that by building a wall,” he told me last year. “The drugs can come under the wall, they can come over the wall, and they can come around the wall.”

That’s where some of the exotic methods of drug smuggling come in: boats, tunnels, drones, submarines, and even homemade bazookas. Over the decades, drug traffickers have proven ingenious at finding alternative methods to sneak by border security, even as the US greatly increased the number of border patrol agents. There’s no reason to think a wall will be an indomitable barrier when the potential for drug profits is so high that the cost of new gadgets or technologies to surpass border security pales in comparison.

Andreas also noted that Trump would not lock down legal points of entry, where commercial vehicles go through every single day — at times carrying hidden drugs. “Much of the cocaine and heroin that comes into the US goes right on the road,” he said, “through the substantial amount of cargo that comes into the United States from Mexico — one of our most important trading partners.”

Even if the US managed to crack down on drugs from Mexico, traffickers would likely just shift their operations to the Caribbean, Canada, or someplace else, Andreas said.

This is an example of the balloon effect: When drug trafficking is stamped out in one area, it just pops up in other places — because drug profits are so lucrative that traffickers are incentivized to always find alternatives, and the drug profits will give them the resources to successfully find those alternatives. This happened in the 1980s and ’90s as federal officials cracked down on trafficking from the Caribbean; trafficking simply moved to Mexico.

“I’m assuming he’s saying his wall is going to solve all sorts of problems simply because that sounds good politically,” Andreas said. “It’s a simple, clear political sound bite. But a wall — the least effective thing it’s going to be for is stopping drug smuggling.”

Besides, many of the drugs that Americans are overdosing on now are homegrown. The current drug epidemic, after all, started when doctors prescribed an absurd amount of painkillers — enough to fill a bottle of pills for every adult in the US in 2012. A wall will do nothing to curtail these legal medications — or illegal substances, such as fentanyl and meth, that are commonly cooked up in domestic labs.

That’s why the most important issue in addressing the opioid epidemic comes down to addressing demand, not supply. If the supply of illicit drugs can’t be reasonably limited, perhaps the demand can be — by providing treatment programs that get opioid users to stop using drugs.

Trump, to his credit, did say that “we will expand treatment for those who have become so badly addicted.” But he has yet to give specifics as to how, exactly, he would accomplish that, while he has repeatedly outlined his plans for border security.

For more on the opioid painkiller and heroin epidemic, read Vox’s explainer.