Nafcillin is a narrow-spectrum antibiotic, similar to penicillin. It is commonly used to treat penicillin resistant staphylococcus, being effective against all strains apart from MRSA.
Like all penicillin class antibiotics, allergies are the most common risk. Other side effects can include hypokalemia, nausea, diarrhea, abdominal pain and yeast infections.
Nafcillin at Wikipedia