Needle exchange programs are an exhaustively studied, proven public health intervention in the fight against opioid addiction — making them a key component in efforts to halt the opioid epidemic. But needle exchanges, it turns out, are also illegal in 15 states.

Take a look at this map by HIV/AIDS advocacy organization amfAR, recently resurfaced by University of Texas Austin clinical assistant professor Lucas Hill:

As the map shows, the programs are banned in most of the South and much of the West.

At the federal level, needle exchanges aren’t banned. But until as recently as 2016 federal dollars were not allowed to go to such programs.

Now, legalization doesn’t necessarily mean that needle exchanges can be easily opened and remain in place. In California’s Orange County, for example, local officials shut down the district’s only needle exchange and have tried to prevent its reopening.

Needle exchanges not only provide a place where people can obtain syringes for drug use or dispose them. They also can help link people to other services, including addiction treatment, vaccinations, and infection testing. The driving idea behind the programs is this: Recognizing the reality that some people do use drugs, it’s better to provide them somewhere to pick up new, sterile syringes (without the risk of spreading, say, HIV or hepatitis C infection) than to turn them away from potential public health services and force them to reuse needles despite the hazards.

The decades of research about needle exchanges is clear: The programs combat the spread of blood-borne diseases like hepatitis C and HIV, cut down on the number of needles thrown out in public spaces, and connect more people to treatment — all without enabling more drug use. This is an exhaustive body of research, backed by independent academic researchers, the World Health Organization, and the Centers for Disease Control and Prevention.

Yet needle exchanges often face resistance due to stigma and concerns that the programs will attract drug users to an area and lead to more drug-related problems. In Orange County, officials in Santa Ana, the county seat, said that the needle exchange program there led to a massive increase in the amount of needle litter in public areas.

Advocates of such exchanges argue that the people using drugs already live in these areas. They are just using them without any place to reliably visit to pick up sterile syringes.

Supporters of needle exchanges also point to studies that show the programs actually reduce drug-related problems such as needle litter.

For example, a 2012 study published in Drug and Alcohol Dependence compared a city with needle exchanges, San Francisco, to one without exchanges, Miami. Through visual inspections, researchers found 44 syringes per 1,000 census blocks in San Francisco, compared with 371 syringes per 1,000 census blocks in Miami. Based on a survey of injection drug users in both cities, the researchers also concluded that users in Miami had more than 34 times the adjusted odds to improperly dispose of syringes in public than users in San Francisco.

“These results suggest that [needle exchanges] are a significant means of collecting used syringes and do not increase the amount of publically discarded used syringes,” the study concluded.

The research, however, often doesn’t penetrate the stigma surrounding addiction. After decades of the United States treating addiction as a criminal justice problem instead of a public health concern, many policymakers and members of the public still view addiction as an issue that demands a punitive response — and see those with addictions as part of the problem instead of as individuals in need of help. (I see this regularly: A common response to just about any opioid story I write is that people dying from drug overdoses are just part of “Darwin’s theory in action.”)

So programs that help populations with addiction are quickly seen as disposable if they are linked, rightly or wrongly, to any problem whatsoever, regardless of the benefits they might produce.

The result has been not only local resistance to needle exchanges, but also outright bans in 15 states.

For more on needle exchanges, read Vox’s explainer on the Santa Ana situation.