[A Case of Suspected Serotonin Syndrome Caused by the Combined Use of Tramadol-acetaminophen Tablets and Fentanyl Patches].

A woman in her 80s was diagnosed with colon cancer, and she underwent chemotherapy after surgery. Oxycodone extended-release tablets were initiated for abdominal pain, but oral administration was difficult, leading to a change to fentanyl patches. Owing to severe numbness from the buttocks to the toes, tramadol/acetaminophen (T/A) tablets were initiated at a dose of 2 tablets per day. The fentanyl patch dose was increased to 2 mg/d, and pain was relieved on combination with 2 T/A tablets. The patient subsequently reported behavioral disturbances, including agitation, anxiety, and sudden diarrhea. She had been taking 1 T/A tablet 2 to 3 times per day at her discretion when the pain worsened. Suspecting an additive effect from the combination of the fentanyl patch and T/A tablets, the T/A tablets were immediately discontinued, and the symptoms resolved. Loxoprofen tablets were initiated at 180 mg/d, and the fentanyl patch dose was increased to 4 mg/d, without side effects such as agitation, anxiety, or behavioral disorders. The combined use of tramadol and fentanyl likely induced excessive serotonergic activity; careful observation is required in adult patients and those receiving multiple medications.