What are the recommended safety risks and liver-monitoring steps if someone insists?

The only human evidence shows fenbendazole can cause severe liver injury with no tumor response. If someone insists on taking it, the monitoring steps are straightforward but the evidence base for them is almost nonexistent.

The single published human case [1] gives you the only concrete numbers. An 80-year-old woman on 1 gram daily (3 days on, 4 off) saw her AST jump from 24 to 386 and ALT from 16 to 487 after 30 days. Her bilirubin rose from 0.8 to 1.3 mg/dL. The Naranjo score of 6 means the drug was a probable cause. Her enzymes normalized after she stopped. That is the entire human safety record. No controlled study has ever measured liver function in fenbendazole users over time, so no one knows what dose or duration is safer, whether pre-existing liver disease raises risk, or whether the injury is predictable.

The veterinary study [2] shows that fenbendazole concentrates heavily in the liver. In sheep, liver levels of the parent drug reached 4,862 ng/g after a solid dispersion formulation, versus 23 ng/g for the base drug. The sulfoxide metabolite hit 18,248 ng/g. That tells you the liver is a primary target organ for accumulation, which makes the hepatotoxicity mechanism biologically plausible. But it does not tell you what human monitoring schedule works.

My call: if someone insists, check AST, ALT, and bilirubin at baseline, then every two weeks while on fenbendazole. Stop immediately if any value doubles the upper normal limit. But the evidence does not support any dose or duration as safe. Confidence: moderate, because the monitoring advice is based on one case and general hepatotoxicity principles, not on any systematic human data.