This article discusses a mice study showing that fenbendazole increases the liver toxicity of acetaminophen (paracetamol)(Tylenol™)
Introduction
Fenbendazole is a drug widely used to treat parasites in animals like dogs, cats, and lab mice. But could it interact dangerously with acetaminophen (Tylenol), a commonly used fever-lowering human medication?
A 2012 study in Toxicological Sciences uncovered surprising risks when these two drugs are combined—a finding with potential implications for humans, especially as fenbendazole gains unproven popularity in alternative health circles.
Key Findings: Fenbendazole Made Tylenol Toxicity Worse in Mice
Liver Damage Skyrocketed
Mice given fenbendazole for a week before receiving a high dose of acetaminophen suffered severe liver injury—far worse than mice given acetaminophen alone.
Blood markers of liver damage (ALT/AST enzymes) spiked higher, and 63% of the mice died within 24 hours when the drugs were combined.
Fenbendazole Alone Was Safe
Mice given only fenbendazole showed no liver harm. The danger arose only when combined with acetaminophen.
Why Did This Happen?
The study ruled out changes in how the liver metabolized acetaminophen. Instead, the problem was glutathione depletion.
Glutathione is a natural antioxidant that neutralizes acetaminophen’s toxic byproducts. Fenbendazole caused glutathione levels to stay dangerously low for longer, leaving the liver unprotected.
Relevance to Humans: Should You Be Concerned?
While this study was done in mice, it raises red flags for humans—especially because:
- Fenbendazole is not approved for human use, but some people take it off-label (e.g., for unproven cancer treatments).
- Acetaminophen (Tylenol) is one of the most widely used painkillers globally. Overdoses are already a leading cause of acute liver failure.
What This Means for You:
- If you’re taking fenbendazole, Avoid combining it with acetaminophen. Even standard Tylenol doses could become riskier.
- Stick to approved medications: Fenbendazole is a veterinary drug with limited human safety data. Its risks—especially drug interactions—are poorly understood.
- Watch for symptoms: Liver damage from acetaminophen can start with nausea, abdominal pain, and jaundice (yellowing skin/eyes). Seek medical help immediately if these occur.
The Science Simplified: How Glutathione Protects Your Liver
When you take acetaminophen, your liver breaks most of it down into harmless substances. However, a small fraction becomes a toxic compound called NAPQI.
Usually, glutathione neutralizes NAPQI. But if glutathione runs out (from overdose or fenbendazole’s effects), NAPQI builds up and destroys liver cells.
Fenbendazole’s Role
- It didn’t change how acetaminophen was processed.
- Instead, it prolonged glutathione depletion, leaving the liver defenseless for longer.
Limitations of the Study
Animal Study
Mice aren’t humans. The doses used were high (300 mg/kg of acetaminophen—far above human therapeutic levels).
Human doses depend on age and weight, but generally, they are 1-2 325 mg tablets every 4 to 6 hours as needed. Roughly, that is 4.6 to 9.2 mg/kg for a 70 kg person.
No Human Data
There is no direct proof that this interaction occurs in people, but the biological mechanism (glutathione depletion) is plausible.
Glutathione (GSH) is a crucial antioxidant in the body that plays a key role in detoxifying acetaminophen (Tylenol) and protecting against liver damage.
The Bottom Line
This study is a cautionary tale: Mixing unapproved drugs with common medications can have unintended consequences.
While fenbendazole might seem harmless, its interaction with acetaminophen highlights how little we know about its safety in humans.
If you’re using fenbendazole:
- Inform your doctor immediately.
- Avoid acetaminophen entirely unless approved by a healthcare provider.
- Prioritize FDA-approved treatments for your condition.
In future articles, I will write about two case reports of people who had elevated liver enzymes secondary to the intake of fenbendazole.
Don’t Get Sick!
Gardner CR, Mishin V, Laskin JD, Laskin DL. Exacerbation of acetaminophen hepatotoxicity by the anthelmintic drug fenbendazole. Toxicol Sci. 2012 Feb;125(2):607-12. doi: 10.1093/toxsci/kfr301. Epub 2011 Nov 1. PMID: 22048645; PMCID: PMC3262853.
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