For nearly three years, in an undisclosed US city, a social service agency has quietly been inviting people to inject illegal drugs at a clandestine site, without the government’s approval.

More than 100 people have injected drugs at the site, according to data released Tuesday about the 2,754 injections. This data provides the first glimpse of what it would look like if the US decided to follow 10 other countries and open supervised injection facilities.

Researchers said two people’s lives were saved because they overdosed at the site, where staff had immediate access to the opioid reversal drug naloxone, and 90% of people who used there said they would have injected in a public restroom, street, park or parking lot if the site had not been available.

“With half a million people dying of overdose since 2000 in the US, we have a huge problem,” Alex Kral, an epidemiologist from RTI International, a nonprofit research group, told the Guardian. “We’ve been trying to do many things about that, but clearly not enough is being done.”

The secret site is modeled after legal facilities in countries including Canada, Denmark and France, where users can take drugs in a safe space with clean supplies. These facilities have been found to provide safety for users as well as communities where used needles can litter the streets, parks and public restrooms.

Kral and Peter Davidson, a medical sociologist at the University of California, provided a rare glimpse into the US facility in an American Journal of Preventative Medicine commentary published Tuesday. For two years, the facility has given an anonymous survey to its clients, who are overwhelmingly male (91.3%), white (80%) and homeless (80.5%).

There are limits to what the facility can offer, and what the data can reveal, because the site is not sanctioned, but the survey responses showed that in the previous 30 days, 25% of the users had witnessed an overdose outside the site and 67.45% had disposed of a syringe in a public place.

Supervised injection sites have long been controversial, but the scale of the opioid addiction crisis in the US – which sees 91 people die from overdoses each day – appears to be shifting attitudes.

Last week the new US surgeon general, Jerome Adams, said supervised injection facilities should be explored as a tool in responding to the addiction crisis. And in June the American Medical Association, the largest body of physicians in the US, endorsed a resolution calling for pilot supervised injection programs.

But in 2014 these facilities were still far from becoming a reality, so the underground injection site was created. “The staff didn’t want to wait around for it to be sanctioned,” Kral said.

The secret site has two rooms, one with five injection stations and the other for post-injection monitoring. It is open four to six hours a day, five days a week and is available by invitation only.

Users spend between 10 and 20 minutes in the room, which is always staffed by at least one person who has been trained in overdose prevention, resuscitation, use of naloxone and injection technique.

The site’s existence, an act of public health civil disobedience, is a throwback to unsanctioned needle exchange sites that appeared during the HIV/Aids crisis. In Canada, where supervised injection sites exist, activists have also created “pop-up” injection tents to respond to the opioid crisis.

Kral said he did not know about any other unsanctioned facilities in the US.

Soon, however, legal facilities could open in Seattle and San Francisco. There are also efforts under way to bring these facilities to other US cities, if lawmakers deem them safe and effective.

New York City approved $100,000 to study how an injection facility would impact the city; Seattle voted to open a site and San Francisco launched a taskforce to study the spaces after research found the city could save $3.5m annually with one 13-booth supervised injection facility. A similar study published in May found Baltimore could save $6m in medical costs if it built one supervised injection facility.

Kral said the unsanctioned facility’s data could help make the case for these sites: “This has been open for almost for three years and the sky has not fallen.”