Nafcillin sodium is a narrow-spectrum beta-lactam antibiotic of the penicillin class. As a beta-lactamase-resistant penicillin, it is used to treat infections caused by Gram-positive bacteria, in particular, species of staphylococci that are resistant to other penicillins. Nafcillin is considered therapeutically equivalent to oxacillin, although one retrospective study found greater rates of hypokalemia and acute kidney injury in patients taking nafcillin compared to patients taking oxacillin.