Nafcillin is an antibiotic in the penicillin group of drugs. It fights bacteria in your body.

Nafcillin is used to treat many different types of infections, especially those caused by staphylococcus bacteria ("staph" infections).

Nafcillin may also be used for other purposes not listed in this medication guide.

You should not use this medication if you are allergic to nafcillin or to any other penicillin antibiotic, such as amoxicillin (Amoxil), ampicillin (Omnipen, Principen), oxacillin (Bactocill), penicillin (Beepen-VK, Ledercillin VK, Pen-V, Pen-Vee K, Pfizerpen, V-Cillin K, Veetids), and others.

Before using nafcillin, tell your doctor if you are allergic to cephalosporins such as Ceftin, Cefzil, Keflex, Omnicef, and others, or if you have asthma, liver or kidney disease, or a history of any type of allergy.

Use this medication for the full prescribed length of time. Your symptoms may improve before the infection is completely cleared. Nafcillin is usually given for up to 2 days after lab tests show that the infection has cleared.

Antibiotic medicines can cause diarrhea, which may be a sign of a new infection. If you have diarrhea that is watery or bloody, stop taking nafcillin and call your doctor. Do not use anti-diarrhea medicine unless your doctor tells you to.

You should not use this medication if you are allergic to nafcillin or to any other penicillin antibiotic, such as:

• amoxicillin (Amoxil, Amoxicot, Biomox, Dispermox, Trimox);

• ampicillin (Omnipen, Principen);

• carbenicillin (Geocillin);

• cloxacillin (Cloxapen);

• dicloxacillin (Dycill, Dynapen);

• oxacillin (Bactocill); or

• penicillin (Beepen-VK, Ledercillin VK, Pen-V, Pen-Vee K, Pfizerpen, V-Cillin K, Veetids, and others).

Before using nafcillin, tell your doctor if you are allergic to any drugs (especially cephalosporins such as Ceftin, Cefzil, Keflex, Omnicef, and others), or if you have:

• asthma;

• liver disease;

• kidney disease; or

• a history of any type of allergy.

If you have any of these conditions, you may need a dosage adjustment or special tests during treatment.

FDA pregnancy category B. This medication is not expected to be harmful to an unborn baby. Tell your doctor if you are pregnant or plan to become pregnant during treatment.

Nafcillin can pass into breast milk and may harm a nursing baby. Do not use this medication without telling your doctor if you are breast-feeding a baby.