In addition, gentamicin should not be used with vancomycin in patients with MRSA NVE because of the nephrotoxicity risk.13,149 In cases of brain abscess com- plicating MSSA IE, nafcillin is the preferred agent rather than cefazolin, which has inadequate blood-brain barrier penetra- bility. If the patient cannot tolerate nafcillin therapy, then van- comycin should be used. Vancomycin is often included with cefazolin as empirical coverage for patients with IE caused by S aureus while await- ing susceptibility results.An analysis of the literature, however, compared the use of empirical combination of vancomycin and antistaphylococcal -lactam therapy with vancomycin alone and demonstrated the superiority of -lactam containing regi- mens over vancomycin monotherapy for bacteremic MSSA infections, including IE.154 This differential outcome included studies in which there was an early shift from empirical van- comycin to -lactam therapy as soon as blood cultures yielded MSSA (not MRSA). The meta-analysis included small, retro- spective studies, however, which limits support for initial com- bination therapy by some experts. Therefore, the usefulness of empiric combination therapy in patients with S aureus bactere- mia until oxacillin susceptibility is known is uncertain. Although the large majority of staphylococci are resis- tant to penicillin, occasional strains remain susceptible. Unfortunately, the current laboratory screening procedures for detecting penicillin susceptibility may not be reliable. Therefore, IE caused by these organisms should be treated with regimens outlined for MSSA that includes nafcillin (or equivalent antistaphylococcal penicillin) as an option rather than penicillin (Table 10). There are no evidence-based data that demonstrate the most appropriate duration of nafcillin therapy for treat- ment of left-sided NVE caused by MSSA. For patients with uncomplicated infection, 6 weeks of therapy is recommended. For patients with complications of IE such as perivalvular abscess ormation and septic metastatic complications, at least 6 weeks of nafcillin is recommended. 