treat endocarditis (infection of the heart chambers). Cautions There is inadequate evidence of safety in human pregnancy. Animal studies and many years of clinical experience suggest that fusidic acid is devoid of teratogenic effects (birth defects), but fusidic acid can cross the placental barrier. Side-effects Fucidin tablets and suspension, whose active ingredient is sodium fusidate, occasionally cause liver damage, which can produce jaundice (yellowing of the skin and the whites of the eyes). This condition will almost always get better after the patient finishes taking Fucidin tablets or suspension. Other related side-effects include dark urine and lighter-than-usual feces. These, too, should normalize when the course of treatment is completed. Patients taking the drug should tell their doctors if they notice these side effects. Resistance In vitro susceptibility studies of U.S. strains of several bacterial species such as S. aureus, including MRSA and coagulase negative Staphylococcus, indicate potent activity against these pathogens. In the UK and Australia, susceptibility is defined as a minimum inhibitory concentration (MIC) of 0.25 mg/L or 0.5 mg/L or less. Resistance is defined as an MIC of 2 mg/L or more. In laboratories using disc diffusion methods, susceptibility for a 2.5 µg disc is defined as a zone of 22 mm or more, and resistance is defined as a zone of 17 mm or less; intermediate values are defined as intermediate resistance. These susceptibility criteria are based on lower dosing regimens used outside of the U.S. Clinical trials in the U.S. incorporate a different dosing regimen that results in higher blood levels. Therefore, the U.S. dosing regimen may warrant different susceptibility criteria. Mechanisms of resistance have been extensively studied only in Staphylococcus aureus. The most studied mechanism is the development of point mutations in fusA, the chromosomal gene that codes for EF-G. The mutation alters EF-G so that fusidic acid is no longer able to bind to it. Resistance is readily acquired when fusidic acid is used alone and commonly develops during the course of treatment. As with most other antibiotics, resistance to fusidic acid arises less frequently when used in combination with other drugs. For this reason, fusidic acid should not be used on its own to treat serious Staph. aureus infections. However, at least in Canadian hospitals, data collected between 1999 and 2005 showed rather low rate of resistance of both methicillin-susceptible and methicillin-resistant to fusidic acid, and mupirocin was found to be the more