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Overview: Nafcillin is an antibiotic used to treat certain bacterial infections. It works by killing bacteria and preventing them from forming their cell wall. The most common side effects are pain, swelling, or irritation at the injection site, and rash. Call your health care provider right away if you have swelling of the face, lips, or throat, trouble breathing, uncontrolled movements, reduced need to pee, nausea or vomiting, watery diarrhea, or fever.

Uses

What is nafcillin used for?

Nafcillin is an antibiotic that is commonly used for bacterial infections caused by certain bacteria.

Nafcillin may not be the right medicine for all bacterial infections, depending on which germs are causing the infection. Bacteria in some locations may have become resistant to nafcillin, which will make it not work as well. 

Nafcillin treats only certain bacterial infections. It does not work for viral infections, such as colds, the flu, or COVID-19, or for other non-bacterial infections. Taking nafcillin when it is not needed can make antibiotics less effective against future bacterial infections.

Nafcillin may also be used for other conditions as determined by your health care provider.

How does nafcillin work (mechanism of action)?

Nafcillin kills certain bacteria by preventing them from being able to create their cell wall.

How is nafcillin supplied (dosage forms)?

Brands and other names

  • Nallpen
  • Unipen

Dosage forms and strengths

  • 1 g, 2g, and 10 g injection
  • 20 mg/mL injection
  • 2 g/100 mL injection

How should I store nafcillin?

Nafcillin is usually given by a health care provider in a hospital or clinic. If you will store nafcillin at home, follow the instructions given by your health care provider. 

Side Effects

What are the most common side effects of nafcillin?

The most common side effects of nafcillin are listed below. Tell your health care provider if you have any of these side effects that bother you.

  • Pain, swelling, and/or inflammation at the site of injection
  • Rash

There may be other side effects of nafcillin that are not listed here. Contact your health care provider if you think you are having a side effect of a medicine. In the U.S., you can report side effects to the FDA at www.fda.gov/medwatch or by calling 800-FDA-1088 (800-332-1088). In Canada, you can report side effects to Health Canada at www.health.gc.ca/medeffect or by calling 866-234-2345.

What are the serious side effects of nafcillin?

While less common, the most serious side effects of nafcillin are described below, along with what to do if they happen.

Severe Allergic Reactions. Nafcillin may cause allergic reactions, which can be serious. Your health care provider will stop using nafcillin and give you emergency treatment if you have any of the following symptoms of a serious allergic reaction.

  • Breathing problems or wheezing
  • Racing heart
  • Fever or general ill feeling
  • Swollen lymph nodes
  • Swelling of the face, lips, mouth, tongue, or throat
  • Trouble swallowing or throat tightness
  • Itching or skin rash
  • Bumps on the skin called hives that can be red, pink, white, or brown depending on your skin tone
  • Nausea or vomiting
  • Dizziness, feeling lightheaded, or fainting
  • Stomach cramps
  • Joint pain

Seizures. Some people who have used nafcillin have reported having seizures. There are many different types of seizures and some of them are not easy to recognize. Stop using nafcillin and call your health care provider if you experience any of the following signs that a seizure is happening or could happen.

  • Muscle twitching, jerking, spasms, tremors, or other uncontrolled movements
  • Stiffening of your arms and legs
  • Feeling anxious, nervous, confused, or a sense of dread
  • Feeling dizzy or lightheaded
  • Falling or losing consciousness

Kidney Damage. Kidney damage can happen when using nafcillin. Call your health care provider right away if you have any of the following symptoms of kidney damage.

  • Reduced need to pee
  • Swelling in your feet, ankles, or legs
  • Weakness or unusual tiredness
  • Trouble catching your breath or chest pain/pressure
  • Confusion
  • Nausea
  • Seizures

Liver Damage. Liver damage, also called hepatotoxicity, can happen when using nafcillin. Call your health care provider right away if you have any of the following symptoms of liver damage.

  • Nausea or vomiting
  • Stomach or belly pain
  • Fever
  • Weakness or unusual tiredness
  • Itching
  • Loss of appetite
  • Light-colored poop
  • Dark-colored urine
  • Your skin or the whites of your eyes turning yellowish in color (also called jaundice)

Antibiotic-Associated Diarrhea. An overgrowth of germs called Clostridioides difficile,or “C. diff,” can happen in your gut with many types of antibiotics, including nafcillin. This can cause a condition known as antibiotic-associated diarrhea and can happen even up to 2 months after stopping the antibiotic. Call your health care provider right away if you have any of the following symptoms of this condition.

  • Watery diarrhea
  • Diarrhea that does not go away
  • Blood in your stool (poop)
  • Severe stomach cramps
  • Fever that starts after you start or finish the medicine

Blood Disorders. Nafcillin can cause blood disorders, such as agranulocytosis, neutropenia, and bone marrow depression. Stop using nafcillin and get help right away if you have any of the following symptoms of blood disorders.

  • Fever
  • Shortness of breath
  • Pale or yellowish skin
  • Easy bruising or bleeding
  • Frequent infection
  • Unusual weakness or tiredness
  • Dizziness, lightheadedness, or feeling like you are about to pass out
  • Headache
  • Fast or abnormal heartbeat

Warnings & Precautions

Who should not use nafcillin?

Allergies to Ingredients. People who are allergic to any of the following should not use nafcillin.

  • Nafcillin
  • Any of the ingredients in the specific product dispensed

Your pharmacist can tell you all of the ingredients in the specific nafcillin products they stock.

What should I know about nafcillin before using it?

Nafcillin is usually given by a health care provider in a hospital or clinic.

If you will store nafcillin at home, do not share it with other people, even if they have the same condition as you. It may harm them. Keep nafcillin out of the reach of children.

What should I tell my health care provider before using nafcillin?

Tell your health care provider about all of your health conditions and any prescription or over-the-counter (OTC) medicines, vitamins/minerals, herbal products, and other supplements you are using. This will help them know if nafcillin is right for you.

Other Medicines and Supplements. Nafcillin may interact with other medicines and supplements. Before using nafcillin, tell your health care provider about any prescription or over-the-counter (OTC) medicines, vitamins/minerals, herbal products, and other supplements you are using. See the Interactions section for more details.

In particular, make sure that you discuss any of the following.

  • An allergy to penicillin, any other medicines, corn, or corn products
  • Asthma
  • Liver disease
  • Kidney disease
  • History of antibiotic-associated diarrhea

Pregnancy. It is not known if or how nafcillin could affect pregnancy or harm a fetus. Tell your health care provider if you are or plan to become pregnant. Your health care provider will let you know if you should use nafcillin while you are pregnant or trying to get pregnant.

Breastfeeding. Nafcillin passes into breast milk. Tell your health care provider if you are breastfeeding or plan to breastfeed. Your health care provider will let you know if you should use nafcillin while breastfeeding.

Interactions

Does nafcillin interact with foods or drinks?

There are no known interactions between nafcillin and foods or drinks.

It is unknown if drinking alcohol will affect nafcillin, but heavy drinking can slow your immune response and make it harder for your body to fight infection.

Does nafcillin interact with other medicines (drug interactions)?

Always tell your health care provider about any prescription or OTC medicines, vitamins/minerals, herbal products, and other supplements you are using.

In particular, make sure that you discuss if you are using any of the following before using nafcillin.

  • A tetracycline, such as tetracycline, doxycycline, minocycline, omadacycline (Nuzyra), or sarecycline (Seysara)
  • Warfarin (Coumadin, Jantoven), which is a blood thinner
  • Cyclosporine (Gengraf, Neoral, Sandimmune, and others), which is a medicine to suppress your immune system
  • Probenecid, which is a medicine commonly used for gout

This may not be a complete list of medicines that can interact with nafcillin. Always check with your health care provider.

Does interact with other drugs you are taking?

Enter your medication into the WebMD interaction checker

Overdose/Missed Dose

What should I do if I accidentally use too much nafcillin?

If you or someone else has used too much nafcillin, get medical help right away, call 911, or contact a Poison Control center at 800-222-1222.

If too much nafcillin has been used, nafcillin will need to be discontinued. A health care provider may treat your symptoms and institute any necessary supportive measures.

What should I do if I miss a dose of nafcillin?

If you miss a dose, use it as soon as you remember. If it is almost time for your next dose, skip the missed dose and only use the next dose. Do not use double or extra doses.

It is important to complete your prescribed course of nafcillin, even if your symptoms get better or go away. This will reduce the risk that future infections are resistant to nafcillin or other similar medicines.