Researchers say people who misuse fentanyl may experience brain damage that causes a temporary loss of memory.

Last May, a 30-year-old man just weeks out of a rehab facility arrived at a West Virginia hospital repeating the same questions over and over, as if he couldn’t remember what he had just asked.

His family had found him in the morning after a late night out with drug paraphernalia around him.

At the hospital, an MRI revealed irregularities in his brain, specifically in the hippocampus, where long-term memories are formed.

With that test, he became the 16th person known to have developed a rare form of amnesia that experts fear is connected to the opioid epidemic.

This week, a new study examined this man’s case and 15 others to see if there are signs that fentanyl is causing a rare form of amnesia.

As the opioid epidemic continues with 115 people in the United States dying every day from overdoses, researchers have found evidence that even those who survive overdoses may be at risk for an unusual form of amnesia.

In the new study, published in the Annals of Internal Medicine, researchers from West Virginia found evidence that the powerful opioid fentanyl mixed with cocaine could injure the brain to the point that people may have difficulty forming new memories.

Fentanyl can be between 50 to 100 times as powerful as heroin. It’s thought to be a major reason that this opioid epidemic has been so deadly.

The study builds on a report from the Centers for Disease Control and Prevention (CDC), which reported that 14 people in Massachusetts received diagnoses with an unusual form of amnesia.

Those cases occurred between 2012 and 2016. All of the patients either tested positive for opioids or had a history of drug use.

The resulting amnesia was temporary, but persisted for five months in one patient and more than one year in two patients.

The CDC reported that researchers speculated one reason could be an overdose diminished or cut off oxygen to the delicate hippocampus region, causing damage.

However, they said that more research was needed to find out if this was an emerging syndrome.

In the new study, researchers looked at two more additional cases of patients who showed signs of this same amnesia.

They also experimented on rats to look for signs that fentanyl in particular can trigger amnesia on its own or with cocaine, a drug that’s been associated with damage to nerve cells.

Dr. David Edwards, chief of pain medicine and assistant professor of clinical anesthesiology at Vanderbilt University Medical Center in Tennessee, explained why fentanyl is such a potent drug.

“Fentanyl is different from morphine in that… it’s fat soluble,” Edwards told Healthline. “Its benefit for that reason is that the onset of action is very fast.”

Edwards said that too much of the drug causes the respiratory system to shut down. This can cause oxygen deprivation that affects the hippocampus.

“The doses in the brain can get really high really fast, so I’m imagining it could occur in these patients where the doses are so high it became kind of anesthetic,” he said.

Marc Haut, PhD, chair of behavioral medicine and psychiatry at West Virginia University, who led the research in the study, said it’s important to learn what exactly is causing this form of amnesia — whether it’s just oxygen deprivation after an overdose or a potent mix of stimulants and opioids that cause brain damage.

“You can have a direct effect from the chemicals on the brain,” he said.

Stimulants can cause damage of key nerve cells, and opioids can diminish the respiratory system to dangerous levels.

Haut said their current hypothesis is that the “additive effect” of the heart or respiration system being significantly compromised produces this particular syndrome.

He also added that what’s particularly worrying is that many more drug users may have this syndrome but aren’t getting diagnosed, because there’s no one who can flag something is wrong.

Haut pointed out it was most often families who first detected the signs of amnesia in the cases included in the report.

“Is this happening more often than we thought?” Haut said. “Because that has a lot of implications for people who survive overdoses for their ability to be successful in treatment and recovery.”