More than a dozen people who used fentanyl, either alone or in combination with stimulants, have suffered severe memory loss, researchers from West Virginia University report.

These cases involved severe short-term memory loss, HealthDay reports. Imaging scans revealed the patients had lesions on the hippocampus, a region of the brain associated with memory. The patients did not recovery quickly, and may never fully regain their short-term memory, according to lead researcher Marc Haut.

“They all have difficulty learning new information, and it’s pretty dense,” Haut said. “Every day is pretty much a new day for them, and sometimes within a day they can’t maintain information they’ve learned.” He added, “Based upon the imaging, I would be surprised if they didn’t have at least some significant memory problems permanently.”

The findings are published in the Annals of Internal Medicine.