This article discusses two case reports of two people who took fenbendazole and later developed liver problems.
Introduction
In recent years, fenbendazole, a common deworming medication for animals, has gained attention on social media as a possible alternative cancer treatment.
However, despite some scientific evidence supporting this claim, no large-scale trials have been made.
Self-medicating with fenbendazole can be dangerous, especially for the liver.
Two recently published medical case reports highlight severe liver damage linked to fenbendazole use. Here’s what happened to two women who took fenbendazole without medical supervision.
Case 1: An 80-Year-Old Woman with Lung Cancer Developed Severe Liver Injury
An 80-year-old woman with advanced lung cancer had been receiving pembrolizumab, an immunotherapy drug. Nine months into her treatment, doctors discovered severe liver dysfunction.
What Was the Cause?
After talking to her and her family, doctors learned that she had been taking fenbendazole for one month, following a social media trend claiming it could help with cancer.
Her Fenbendazole Dosage:
- 1 gram per day
- Taken for 3 days, then 4 days off
- Repeated for about one month
Outcome:
- Her liver dysfunction resolved after stopping fenbendazole, confirming it was the likely cause of her liver injury.
- Unfortunately, her cancer did not shrink during fenbendazole use but increased in size when it was discontinued.
How Did Doctors Confirm Fenbendazole Was the Cause?
They used the Naranjo Adverse Drug Reaction Probability Scale, a tool that helps determine if a drug is responsible for an adverse reaction. The score of 6 points meant it was a “probable” cause of her liver injury.
Comment: Pembrolizumab can cause liver toxicity in <1% during its clinical trials.[2]
Another paper said that Pembroluzimab could cause mild-to-moderate serum aminotransferase elevations in 20% to 30% during pembrolizumab therapy but are usually self-limited and resolve even with continuing cyclic therapy.[3]
Serum ALT elevations above 5 times the upper limit of normal (ULN) occur in 1% to 4% of patients and generally lead to temporary discontinuation. Importantly, in 1% to 2% of patients, the serum enzyme elevations evolve into an immune-mediated liver injury that can be clinically apparent and can be severe. The onset of injury is usually after 2 to 4 cycles or 1 to 3 months after starting treatment. [3]
The pattern of liver injury in the case above reflects the pattern seen in pembrolizumab liver toxicity.
Incidentally, in the Conflicts of Interest statement in their case report, one of the authors (T.H.) works with Merck, which manufactures pembrolizumab. [1]
Case 2: A 67-Year-Old Woman with a History of Colon Cancer Developed Severe Liver Damage
A 67-year-old woman who had undergone surgery for colon cancer went to the hospital with:
❌ Jaundice (yellow skin and eyes)
❌ Nausea and loss of appetite
❌ Dark urine
❌ A painful rash
She had no history of liver disease but drank two glasses of wine daily and had recently taken antibiotics.
The Investigation:
- Doctors tested for hepatitis, autoimmune diseases, and other liver conditions, but all tests were negative.
- A liver biopsy (a sample of liver tissue) showed massive liver cell damage with an inflammatory reaction.
- When questioned further, she admitted taking fenbendazole for the past year—three 1g packets three times a week for a precancerous skin lesion.
- Her last dose was 4 days before hospitalization.
Outcome:
- Doctors diagnosed her with severe drug-induced liver injury (DILI) due to fenbendazole.
- Her bilirubin (a marker of liver function) peaked at 24 mg/dL—a dangerous level.
- She was treated with budesonide (a steroid) and supportive care.
- Her liver function improved over the next three months after stopping fenbendazole.
How Did Doctors Confirm Fenbendazole Was the Cause?
They used the Roussel Uclaf Causality Assessment Method (RUCAM), a standard test for drug-induced liver injury. Her score of 9 points meant fenbendazole was highly likely to be the cause.
Comment: In case 2, the patient admits to drinking two glasses of wine daily. In medical school, a frequent adage is that some patients underreport their alcohol use. A glass of wine is 250 ml. A bottle is 750 ml. I would say that someone who will save the last 250 ml for another day has an astounding self-discipline.
Tylenol (Acetaminophen), in combination with alcohol, can cause severe liver disease.
What This Means for You
Fenbendazole is NOT a proven cancer treatment.
There is no strong scientific evidence showing that fenbendazole cures or slows down cancer in humans. The cases above show serious liver damage, and in neither case did the cancer improve.
The liver is highly sensitive to medications, alcohol, and supplements.
The liver filters toxins and drugs, and some substances—like fenbendazole and acetaminophen (Tylenol)—can be toxic in large doses or over long periods.
Social media is NOT a medical source.
Both women started taking fenbendazole after hearing about it on social media without discussing it with their doctors. Many online claims lack scientific backing and can be dangerous.
If you decide to use fenbendazole for whatever reason, do your research and get it from reputable sources, and observe and double-check if it can interfere with other medications, prescription, and over-the-counter supplements.
Takeaways:
✅ Always talk to your doctor before taking any new drug or supplement—especially for cancer.
✅ Self-medicating with animal drugs like fenbendazole can cause serious liver damage.
✅ If you are on cancer treatment, adding unapproved medications can make things worse.
✅ Scientific studies—not social media—should guide your health decisions.
My impression: The two case reports presented here provide valuable insights into the potential for fenbendazole to cause liver injury. However, the tone of the writing suggests that they are hit pieces against fenbendazole and social media.
There are still no large-scale trials, and it probably will never happen. The use of fenbendazole will remain a personal decision for cancer patients.
First, chemotherapy drugs can cause an increase in liver enzymes. Second, Consuming two glasses of wine daily, like in the second case, could also increase liver enzymes.
If someone decides to use fenbendazole, they must stay in touch with their physicians to monitor their liver enzymes.
💡 If you or a loved one are considering alternative cancer treatments, always consult a medical professional first.
Don’tGet Sick!
- Yamaguchi T, Shimizu J, Oya Y, Horio Y, Hida T. Drug-Induced Liver Injury in a Patient with Non-small Cell Lung Cancer after the Self-Administration of Fenbendazole Based on Social Media Information. Case Rep Oncol. 2021 Jun 17;14(2):886-891. doi: 10.1159/000516276. PMID: 34248555; PMCID: PMC8255718.
- Kwok G, Yau TC, Chiu JW, Tse E, Kwong YL. Pembrolizumab (keytruda) Hum Vaccin Immunother. 2016;12((11)):2777–89. doi: 10.1080/21645515.2016.1199310.
- LiverTox: Clinical and Research Information on Drug-Induced Liver Injury [Internet]. Bethesda (MD): National Institute of Diabetes and Digestive and Kidney Diseases; 2012-. Pembrolizumab.
- Thakurdesai A, Rivera-Matos L, Nagra N, Busch B, Mais DD, Cave MC. Severe Drug-Induced Liver Injury Due to Self-administration of the Veterinary Anthelmintic Medication, Fenbendazole. ACG Case Rep J. 2024 May 2;11(5):e01354. doi: 10.14309/crj.0000000000001354. PMID: 38706451; PMCID: PMC11068125.
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