Because of its limited use, the effects of rifapentine on the liver have been less well defined than those of rifampin, but they are likely to be similar. Thus, long term therapy with rifapentine is associated with minor, transient elevations in serum aminotransferase levels in 2% to 7% of patients, abnormalities that usually do not require dose adjustment or discontinuation. Clinically apparent liver injury due to rifapentine has not been reported, but it is likely to be similar to rifampin in its potential for causing acute liver injury. Because rifapentine is usually given in combination with isoniazid and/or pyrazinamide, two other known hepatotoxic agents, the cause of the acute liver injury in patients on rifapentine containing regimens may be difficult to relate to a single agent, and some evidence suggests that these combinations are more likely to cause injury than the individual drugs. Typically, the onset of injury due to rifamycins is within 1 to 6 weeks and the serum enzyme pattern is usually hepatocellular at the onset of injury, but can cholestatic and mixed in contrast to isoniazid and pyrazinamide. Extrahepatic manifestations due to rifamycin hepatotoxicity such as fever, rash, arthralgias, edema and eosinophilia are uncommon as is autoantibody formation. This potential for hepatotoxicity has not been specifically demonstrated for rifapentine.