They arrive on their own, worried about what was really in that bag of heroin. Some are carried in, slumped between two friends. Others are lifted off the sidewalk or asphalt of a nearby alley and rolled in a wheelchair to what's known as SPOT, or the Supportive Place for Observation and Treatment, at the Boston Health Care for the Homeless Program (BHCHP).

Nine reclining chairs have been full most days, especially during peak mid-day hours. It may be the only room in the country when patients can ride out a heroin or other high under medical supervision.

"It’s a safe place to be," says Tommy, 39, who's been using heroin for at least 21 years. "It’s a lot safer than being out on the street, possibly walking into traffic. I might OD if I was alone out there."

Tommy is looking for a job and housing and we've agreed not to use his full name. He's one of 129 men and women who've come to this former conference room to ride out an opiate or other drug high since SPOT opened seven weeks ago. Nurses have logged 447 visits. At least half of the patients have come more than once.

If the person can speak, a nurse will ask what they took before settling them in a chair, wrapping a blood pressure cuff around one arm and placing an oxygen monitor over a finger.

"The monitors are really convenient," Dr. Jessie Gaeta, BHCHP medical director, says as she pulls the Velcroed edges of a blood pressure cuff apart. "It takes a lot of the guessing out of understanding how far someone is into an overdose syndrome."

The phrase "overdose syndrome" is new. Gaeta made it up to describe what’s happening to patients in this room. It's not a typical overdose.

Dr. Jessie Gaeta, chief medical officer at the Boston Health Care for the Homeless Program, inside the conference room where heroin users are monitored while they ride out a high. (Jesse Costa/WBUR)

"A classic opiate overdose is characterized by a person who stops breathing," Gaeta says. "They have central nervous system depression. So it’s mostly respiratory depression and respiratory arrest."

But that’s not what Gaeta tracks on the monitors connected to about 75 percent of her SPOT room patients.

"What we’re seeing in this room, as we’re watching this happen progressively, is more depression of heart rates and blood pressures as actually the primary, sometimes the only abnormalities," Gaeta says.

Patients tell Gaeta they may start the day with heroin or another opioid and then, a few hours later, take pills that will enhance or extend the high. That helps explain why SPOT's peak hours are noon to 3 or so, and it fits overdose data collected by Boston EMS. A typical combination or cocktail of four drugs: heroin or another opioid, Clonidine (which lowers blood pressure), Klonopin (to control for anxiety) and Gabapentin (used to treat seizures or nausea). Patients may be prescribed these pills or buy them on the street.

"People are talking about that a lot here," Gaeta says, "about the layering of this cocktail of medications and that’s really reflected in the vital signs that we’re seeing, which is not indicative of pure opiate overdoses. I’m not sure that we’d have seen that without doing this kind of monitoring."

This is just a snapshot based on a small number of addiction patients in one area of Boston. But what Gaeta is seeing in real time is reflected in overdose death reports and is changing the way Gaeta and her staff respond to these patients who look like they are falling into a deep sleep. To boost sinking blood pressures, for example, they've brought IV fluid equipment into the room. And there have been other surprises.

"What you see subjectively looking at someone and what their vital signs are, don’t always match up," nurse April Donahue says.

She says some patients in the room appear so sedated that they don’t respond when she speaks loudly in their ear or raps on their sternum.

"We have people that are difficult to wake up to that point that [they] still have rock solid vital signs, better than mine," Donahue says with a chuckle.

If Donahue was not monitoring these rock solid vital signs, she says she’d be racing to inject naloxone, brand name Narcan, the drug that flushes opioids out of the nervous system. But naloxone may not be what many of these patients need and, Gaeta says, patients hate it.

A nurse prepares seating for drug users in the Supportive Place for Observation and Treatment, or SPOT, at BHCHP. (Jesse Costa/WBUR)

"People sort of wake up vomiting, extremely agitated, sometimes hallucinating, maybe having a bowel movement in the moment, it’s not pretty," Gaeta says. "People don’t want Narcan to be used on them unless it’s absolutely necessary. I mean, they also don’t want to die."

Doctors and nurses at BHCHP used naloxone on five patients in just one day in April, the Friday before SPOT opened. But they've only given it to four patients total in the seven weeks since. Instead they’re using lots of oxygen from big green tanks they roll over to each patient's chair, inserting nasal tubes to keep patients at a normal breathing rate.

Patients given naloxone often bolt from a clinic of emergency room in search of more drugs. Gaeta says a person who comes off a high more naturally may be more open to talking about their substance use, going into treatment or getting tested for HIV, Hepatitis C and other medical problems.

"I think what’s struck me the most is the gratitude," Donahue says, remembering patients who look at her with trust instead of fear or shame. These men and women are tired of having to hide their drug use in an alley or fast food restaurant bathroom.

"Just to get out of that environment, even for a little while, to get off the street and be cared for," Donahue says. "I mean, so many of our participants don’t have anyone who’s caring for them."

It’s one reason Tommy is becoming a repeat client. He’s on methadone, which helps him stay off opioids some days, and dive into one of the books he loves. A worn copy of "Fear and Loathing at Rolling Stone: The Essential Writing of Hunter S. Thompson," with a broken black spine, lies on the table in front of him.

"This is just a great start," Tommy says. "I think it will slow down a lot of overdoses and could save a lot of lives. It will save a lot of lives in the long run."

And Tommy knows. He went into respiratory failure a few weeks ago while at SPOT and was brought back with naloxone.

Boston police and Mayor Marty Walsh's office say no problems have been reported related to this room, where addiction patients can ride out a high under medical supervision.

BHCHP is conducting surveys of the area within 500 meters of the room for evidence of increased or decreased drug use. Those results are expected in the early fall.