http://livertox.nih.gov/Tadalafil.htm
Hepatotoxicity
Despite fairly extensive use, tadalafil has been linked to only rare reports of serum aminotransferase elevations and clinically apparent liver injury. In a single case report, tadalafil was linked to cholestatic hepatitis arising within a few days of starting the medication. Immunoallergic features and autoantibodies were not present. The injury was self-limiting without residual evidence of bile duct injury. The related PDE5 inhibitor, sildenafil, has also been associated with rare cases of acute cholestatic liver injury with jaundice.
Mechanism of Injury
The mechanism of liver injury from tadalafil is unknown, but it is metabolized in the liver via the cytochrome P450 system (CYP 3A4) and a toxic or immunogenic intermediate may account for the rare instances of hepatic injury linked to its use.
Outcome and Management
The liver injury linked to tadalafil has been self-limited and resolving within 2 to 3 months. There is no known cross-sensitivity between tadalafil and the other PDE5 inhibitors currently in use in the United States, but switching to another PDE5 inhibitor should be done with caution.