Nafcillin is a semisynthetic penicillin derived from 6-aminopenicillanic acid. It is normally formulated as a sodium salt with the chemical name Monosodium (2S,5R,6R)-6-(2-ethoxy-1-naphthamido)-3,3-dimethyl-7-oxo-4-thia-1-azabicyclo[ 3.2.0 ] heptane-2-carboxylate monohydrate, molecular formula C21H21N2NaO5S • H2O, molecular weight 454.48.

Nafcillin's bactericidal action is very similar to penicillin, but it is also resistant to inactivation by staphylococcal penicillinase and is effective against both penicillinase and non-penicillinase producing strains of staphylococcus aureus. However, nafcillin should not be used to treat infections caused by bacteria susceptible to penicillin G. Although nafcillin may be used to begin treatment if an infection is suspected to be caused by a resistant staphylococcus, susceptibility tests should always be performed and treatment discontinued if testing shows the infection to be due to any organism other than resistant staphylococcus.

Like penicillin, nafcillin can cause severe and sometimes fatal anaphylactic reactions. Serious reactions may require immediate treatment with epinephrine as well as oxygen, intravenous steroids, and airway management. Nafcillin is primarily cleared out of the body through the liver, so care must be taken when treating patients with hepatic insufficiency and renal dysfunction. Concurrent use of nafcillin and tetracycline should be avoided because tetracycline may antagonize the bactericidal effect of penicillins.

Source: Product insert for Nafcillin Injection, USP