Uses

• Veterinary Use: Treats and prevents gastrointestinal and respiratory parasites in animals, including roundworms, hookworms, whipworms, pinworms, and Giardia. 
• Human Off-Label Use: Although primarily an animal medication, it is widely discussed on social media and internet forums as an off-label treatment for various cancers. Preclinical lab studies indicate it may interfere with cancer cell growth and glucose metabolism, though there is currently no randomized clinical evidence in humans to prove its efficacy or safety for oncology. 
• Other Human Uses: In some countries, similar benzimidazole drugs are prescribed to treat rare, specific parasitic or infectious diseases like filariasis.

Side Effects

• Common: Nausea, vomiting, diarrhea, abdominal pain, and headache.
• Less Common: Dizziness, vertigo, fever, fatigue, and hair loss.
• Rare/Serious: Elevated liver enzymes (hepatotoxicity), bone marrow suppression, and skin rashes. 

Warnings

• Not FDA Approved for Humans: The FDA has not evaluated or approved fenbendazole for human consumption or cancer treatment. 
• Pregnancy Risk: It can be harmful to an unborn baby. It must not be taken during pregnancy or while breastfeeding unless explicitly directed by a doctor. 
• Liver Damage: People with underlying liver conditions should use it with extreme caution, as several case reports have linked off-label human use of the drug to acute, clinically apparent liver injury.
• Drug Interactions: It may interact unpredictably with other medications or substances (such as milk)