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Nafcillin (injection)

Generic name: nafcillin (injection) [ naf-SIL-in ]
Brand names: Unipen, Nallpen
Dosage forms: injectable powder for injection (1 g; 10 g; 2 g), intravenous solution (1 g/50 mL; 2 g/100 mL)
Drug class: Penicillinase resistant penicillins

Medically reviewed by on Feb 8, 2024. Written by Cerner Multum.

What is nafcillin?

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

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

Nafcillin side effects

Get emergency medical help if you have signs of an allergic reaction: hives, itching; fever, stomach pain, muscle or joint pain; difficulty breathing; swelling of your face, lips, tongue, or throat.

A delayed allergic reaction to nafcillin can occur as few as 2 days to as many as 4 weeks after you use the medication.

Nafcillin may cause serious side effects. Call your doctor at once if you have:

Common side effects of nafcillin may include:

This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.


You should not use nafcillin if you are allergic to any penicillin antibiotic.

Before taking this medicine

You should not use this medicine if you are allergic to nafcillin or similar antibiotics, such as:

Tell your doctor if you have ever had:

It is not known whether this medicine will harm an unborn baby. Tell your doctor if you are pregnant or plan to become pregnant.

Nafcillin can pass into breast milk and may harm a nursing baby. Tell your doctor if you are breast-feeding a baby.

How is nafcillin given?

Follow all directions on your prescription label and read all medication guides or instruction sheets. Use the medicine exactly as directed.

Nafcillin is given as an infusion into a vein, or injected into a muscle. A healthcare provider will give your first dose and may teach you how to properly use the medication by yourself.

Infusion into a vein can take 30 to 60 minutes to complete.

Read and carefully follow any Instructions for Use provided with your medicine. Ask your doctor or pharmacist if you don't understand all instructions.

Nafcillin powder must be mixed with a liquid (diluent) before using it.

If you inject into a muscle, give the injection right away after mixing the powder and diluent.

If you inject into a vein, you may store the mixture at room temperature for up to 3 days, or in a refrigerator for up to 7 days. Do not use if the mixture has changed colors or has particles in it. Call your pharmacist for new medicine.

Store unmixed nafcillin powder at room temperature away from moisture and heat.

Your medicine may also be supplied as a premixed solution that has been frozen in a plastic container.

Store frozen nafcillin in a deep freezer, at -4 degrees Fahrenheit (-20 degrees Celsius).

Take the medicine out of the freezer and let it reach room temperature before injecting your dose. You may also thaw the medicine in a refrigerator. Do not use heat to thaw frozen nafcillin.

Use nafcillin for the full prescribed length of time, even if your symptoms quickly improve. Skipping doses can increase your risk of infection that is resistant to medication. Nafcillin will not treat a viral infection such as the flu or a common cold.

Use a needle and syringe only once and then place them in a puncture-proof "sharps" container. Follow state or local laws about how to dispose of this container. Keep it out of the reach of children and pets.

What happens if I miss a dose?

Use the medicine as soon as you can, but skip the missed dose if it is almost time for your next dose. Do not use two doses at one time.

What happens if I overdose?

Seek emergency medical attention or call the Poison Help line at 1-800-222-1222.

Overdose symptoms may include agitation, confusion, hand tremors, hallucinations, seizure, or coma.

What should I avoid while using nafcillin?

Antibiotic medicines can cause diarrhea, which may be a sign of a new infection. If you have diarrhea that is watery or bloody, call your doctor before using anti-diarrhea medicine.

What other drugs will affect nafcillin?

Tell your doctor about all your other medicines, especially:

This list is not complete. Other drugs may affect nafcillin, including prescription and over-the-counter medicines, vitamins, and herbal products. Not all possible drug interactions are listed here.

Further information

Remember, keep this and all other medicines out of the reach of children, never share your medicines with others, and use this medication only for the indication prescribed.

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.