In a 2016 study, tramadol and fentanyl were found to have a higher proportion of psychotic presentations than the other opioids. Despite the fact that there were only seven cases of psychosis with these medicines in the same study, two of these were single-agent cases involving fentanyl and one with tramadol. Only one previous published report of psychosis as a clinical characteristic of tramadol overdose has been reported, and none has involved fentanyl.