Rare Brain Toxicity Linked to Common Chemo Drug 5-FU

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A new case report published in the journal Oncoscience describes a rare and potentially life-threatening complication called hyperammonemic encephalopathy, which is linked to the widely used chemotherapy drug 5-fluorouracil (5-FU). The case involves a 63-year-old woman with pancreatic cancer who developed repeated episodes of confusion and lethargy after receiving 5-FU, despite normal liver function. Upon discontinuation of 5-FU and administration of standard treatments for hyperammonemia, the patient's neurological symptoms resolved rapidly.

Why it matters

This case highlights the importance of monitoring neurological symptoms in cancer patients receiving 5-FU, even when liver function appears normal. Hyperammonemic encephalopathy can be a serious and potentially life-threatening complication, and early recognition and intervention are crucial to prevent serious outcomes.

The details

The patient's past medical history included epileptic seizures, diabetes mellitus, and hypothyroidism. Laboratory tests revealed significantly elevated serum ammonia levels, despite normal liver function. The researchers considered other medications the patient was taking, including antiepileptic drugs and irinotecan, which have also been associated with hyperammonemia. However, the pattern of symptom recurrence following each 5-FU infusion, and resolution after discontinuation, reinforced 5-FU as the primary trigger.

• The case report was published in Volume 12 of Oncoscience on December 23, 2025.
• The patient developed repeated episodes of confusion and lethargy after receiving several cycles of a chemotherapy regimen that included 5-FU.

What’s next

The authors advise careful consideration before reintroducing 5-FU and recommend doing so only in close collaboration with specialists in metabolic or drug-related toxicities.