Using fentanyl or other opioids alongside other illicit drugs could trigger possibly permanent amnesia caused by brain damage, doctors warn. Over a dozen cases have emerged in which drug abusers have developed severe short-term memory loss, possibly after experiencing an overdose, according to a study published in Annals of Internal Medicine.

Imaging scans of patients revealed lesions on the hippocampus, a region of the brain associated with memory. These amnesiac drug users do not recover quickly, and there's some question whether they will ever fully regain their short-term memory, Authors noted

"Based upon the imaging, I would be surprised if they didn't have at least some significant memory problems permanently," said Marc Haut, chair of West Virginia University's department of behavioral medicine and psychiatry. .

The latest case occurred in May 2017, when doctors at a West Virginia hospital treated a 30-year-old Maryland man suffering from persistent memory impairment. Family members reported that the man had a history of heroin use, and had recently left a residential addiction treatment program.

Blood tests revealed the presence of cocaine in his system and urine tests detected norfentanyl, a chemical produced by the breakdown of fentanyl in the body, the researchers said. Meanwhile, imaging scans showed lesions on the man's brain, along the hippocampus and basal ganglia.

Digging further, the investigators found another similar case of drug-related amnesia in Virginia in September 2015, as well as a total of 14 cases between 2012 and 2016 in Massachusetts. None of these previous amnesiac patients were tested for fentanyl, but 15 of the total 16 known cases tested positive for opioid use and half had a history or tests indicating cocaine use.

But, Haut noted, the researchers aren't sure what's causing the brain damage revealed by imaging scans. It's possible that these patients experienced an undetected drug overdose that temporarily stopped their heart or lungs, cutting the flow of oxygen to their brain.

"You get that cutoff of oxygen and that can produce lesions like this, but not to this extent typically," Haut said. "We think the fentanyl is adding to that effect and exacerbating that effect," possibly when taken in combination with a stimulant like cocaine.

Overdose risk is extremely high with fentanyl, which is 50 to 100 times more potent than morphine. That makes the synthetic opioid much more powerful than either heroin or other prescription painkillers, according to the U.S. National Institute on Drug Abuse.

"As the fentanyl starts being combined with heroin, it's creating a huge increase in accidental overdoses," said Dr. Tim Brennan, director of the Addiction Institute at Mount Sinai West and Mount Sinai St. Luke's Hospitals in New York City. The authors hope the report will prompt doctors to take a closer look at patients who come in for either amnesia or drug overdose.

"We talk a lot about people who don't survive overdoses, but we aren't talking about people who survive repeated overdoses and the impact that might have on them and their functioning," Haut said. "If their memory is really compromised, it's going to be hard for them to learn a new life that doesn't involve drugs."