Her mother was supposed to go home the next day.

Sheree Thein’s mom, Jadeen Rivard, was hospitalized in 2014 with a partial intestinal blockage. She had her stomach pumped and after a few days she was feeling better. “There was nothing going on with her that gave us any reason for concern,” Thein said.

Her mom didn’t survive the night.

Thein said her mother was a victim of what’s called “Dead in Bed,” a phenomenon that’s well-known within the medical community, but not discussed much with the general public.

The hospital could offer no answers and an autopsy was inconclusive. But the exam didn’t include a toxicology report and Thein’s family became convinced that if it had, it would have shown that her mother died of opioid intoxication from Dilaudid, the common but potent painkiller she was given during her hospitalization.

Experts who reviewed her medical records said the doses seemed unusually high for someone being treated for such a minor procedure. Cumulatively, they said, it could have been enough to make a patient stop breathing, especially someone like Rivard who was 80 and hadn’t been on opioids before.

It usually happens in hospital wards that aren’t intensive care units. Patients on painkillers, often recovering from surgery, quietly suffer respiratory failure while asleep.

Frank Overdyk, an anesthesiologist and national patient safety expert, said an estimated 3,000 to 5,000 Americans die that way every year.

That’s far fewer than die in hospitals of other complications like infections or surgical errors, but Overdyk said it’s still too many. “They are completely preventable.”

Overdyk — who receives payments from medical device company Medtronic — is chairman of a coalition of 15 groups pushing for continuous electronic monitoring of all hospitals patients on opioids that are injected or infused intravenously.

Very few hospitals do that, though it would alert staff if a patient was struggling to breathe. But other physicians who don’t have financial interests also said the public should be aware of the dangers of overuse of painkillers.

“It’s not just the elderly and it’s not just Dilaudid,” said Andrew Kolodny, the executive director of Physicians for Responsible Opioid Prescribing. “I believe that opioid overdose deaths in hospital beds are not uncommon.”

Hospitals have known of the problem since at least August 2012, when the Joint Commission released an alert about it. It estimated that 47 percent of the adverse events, including deaths, related to inpatient opioids were because of dosing errors. But 29 percent were due to improper monitoring.

Thein has made it her mission to improve patient monitoring and also spread the word about what other families should look for. “I don’t think it’s limited to that hospital at all,” Thein said.