A neurologist in Massachusetts noticed the pattern about a year ago. He had seen four patients come in with amnesia—three who tested positive for opioids (heroin or prescription painkillers) and a fourth with a history of heroin use. Memory loss from drug use is not unusual, but when he peered inside their brains with an MRI scan, he saw something really strange.

All four patients had the same problem: Little or no blood was flowing to their hippocampi, two slivers of tissue deep in brain, one on the left and one right, involved in memory. And the effect was symmetrical. Whatever had damaged the brains of these patients seemed to specifically attack the hippocampus neurons. With their hippocampi impaired, the patients couldn’t form new memories. This is not how opioids are supposed to work. Could this be a rare undiscovered effect from existing opioids or a new effect of a novel synthetic drug?

The neurologist, Jed Barash, then at Lahey Hospital and Medical Center, wrote up the four cases and reported them to the Massachusetts Department of Public Health. This prompted doctors in the state to dig through their patient records for more cases of amnesia with a history of opioid use.

Eventually, they found 10 more, bringing the total to 14 in the last several years in Massachusetts. Doctors were only able to follow up with MRIs in three of the cases; one patient’s hippocampi recovered to normal, but the other two others continued to have shrunken hippocampi. The results were published last week in the CDC’s Morbidity and Mortality Weekly Report. “We wanted to put it to wider attention,” says Alfred DeMaria, Jr., an epidemiologist with the Massachusetts Department of Public Health. He hopes that more doctors will look out for the phenomenon, so they can get to the bottom of it.