Reviewed on 5/5/2022

What Is Nafcillin and How Does It Work?

Nafcillin is a prescription medicine used to treat a wide variety of bacterial infections including infections caused by penicillinase-producing staphylococci.

  • Nafcillin is available under the following different brand names: Nafcil, Nallpen

What Are Dosages of Nafcillin?

Adult and pediatric dosage

Injectable solution

  • 20mg/mL
  • 2g/100mL

Powder for injection

  • 1g
  • 2g
  • 10g

Susceptible Infections

Adult dosage

  • 500 mg IV/IM every 4-6 hours

Pediatric dosage

  • 50-100 mg/kg/day IV/IM divided every 6 hours  

Maximum 12g/day

Severe Infections

Adult dosage

  • 1 g IV/IM every 4 hours

Pediatric dosage

  • 100-200 mg/kg/day IV/IM divided every 6 hours

Staphylococcal Endocarditis

Adult dosage

  • 2 g IV every 4 hours for 4-6 weeks; may be longer and may add rifampin and gentamicin if prosthetics present

Pediatric dosage

  • 200mg/kg/day IV divided every 4-6 hours for 6 weeks; may be longer and may add rifampin and gentamicin if prosthetics present 

Neonates (less than 28 Days Old)

  • (Less than 7 days old, less than 2 kg) OR (more than 7 days old, less than 1.2 kg): 50 mg/kg/day IV/IM divided every 12 hours  
  • (Less than 7 days old, more than 2 kg) OR (more than 7 days old, 1.2-2 kg): 75 mg/kg/day IV/IM divided every 8 hours
  • (more than 7 days old, more than 2 kg): 100-140 mg/kg/day IV/IM divided every 6 hours

Bursitis septic

Adult dosage

  • 2 g IV every 4 hours

Dosage Considerations – Should be Given as Follows: 

  • See “Dosages”

What Are Side Effects Associated with Using Nafcillin?

Common side effects of Nafcillin include:

Serious side effects of Nafcillin include:

  • joint/muscle pain, 
  • signs of kidney problems: such as a change in the amount of urine, pink/bloody/frothy/dark/cloudy urine, extreme tiredness, easy bruising/bleeding, 
  • new signs of infection: such as sore throat that doesn't go away, fever,
  • seizures, 
  • uncontrolled movements, 
  • confusion, 
  • irregular heartbeat, 
  • muscle cramps, and
  • weakness.

Rare side effects of Nafcillin include:

  • none 
This is not a complete list of side effects and other serious side effects or health problems that may occur as a result of the use of this drug. Call your doctor for medical advice about serious side effects or adverse reactions. You may report side effects or health problems to FDA at 1-800-FDA-1088.

What Other Drugs Interact with Nafcillin?

If your medical doctor is using this medicine to treat your pain, your doctor or pharmacist may already be aware of any possible drug interactions and may be monitoring you for them. Do not start, stop, or change the dosage of any medicine before checking with your doctor, health care provider, or pharmacist first.

  • Nafcillin has severe interactions with the following drugs:
  • Nafcillin has serious interactions with at least 107 other drugs.
  • Nafcillin has moderate interactions with at least 170 other drugs.
  • Nafcillin has minor interactions with at least 32 other drugs. 

This information does not contain all possible interactions or adverse effects. Visit the RxList Drug Interaction Checker for any drug interactions. Therefore, before using this product, tell your doctor or pharmacist about all the products you use. Keep a list of all your medications with you and share this information with your doctor and pharmacist. Check with your health care professional or doctor for additional medical advice, or if you have health questions or concerns.

What Are Warnings and Precautions for Nafcillin?


  • A history of a hypersensitivity (anaphylactic) reaction to any penicillin
  • Solutions containing dextrose in patients with known allergy to corn or corn products

Effects of drug abuse

  • None

Short-Term Effects

  • See “What Are Side Effects Associated with Using Nafcillin?”

Long-Term Effects

  • See “What Are Side Effects Associated with Using Nafcillin?”


  • Evaluate renal, hepatic, hematologic systems periodically during prolonged treatment
  • Serious and occasionally fatal hypersensitivity (anaphylactic) reactions were reported; reactions are more likely to occur in individuals with a history of penicillin hypersensitivity and/or a history of sensitivity to multiple allergens; inquire about previous hypersensitivity reactions to penicillins, cephalosporins, or other allergens before initiating therapy; if allergic reaction occurs, discontinue treatment and institute appropriate therapy
  • Use with caution in patients with histories of significant allergies and/or asthma; whenever allergic reactions occur, withdraw penicillin unless, in the opinion of the physician, the condition being treated is life-threatening and amenable only to penicillin therapy
  • The liver/biliary tract is the primary route of nafcillin clearance; exercise caution when patients with concomitant hepatic insufficiency and renal dysfunction are treated with this drug
  • Prescribing the antibiotic in the absence of a proven or strongly suspected bacterial infection or a prophylactic indication is unlikely to provide benefit to the patient and increases the risk of development of drug-resistant bacteria
  • The use of antibiotics may result in the overgrowth of nonsusceptible organisms; if new infections due to bacteria or fungi occur, discontinue the drug and take appropriate measures
  • To optimize therapy, determine causative organisms and susceptibility; more than 10 d treatment to eliminate infection and prevent sequelae (eg, endocarditis, rheumatic fever); take cultures after treatment to confirm that infection is eradicated
  • Elevation of liver transaminases and/or cholestasis may occur, specifically with the administration of high doses
  • Urinalysis, serum blood urea nitrogen, and creatinine determinations should be performed at baseline and periodically during therapy; serum bilirubin, aspartate aminotransferase (AST), alanine aminotransferase (ALT), alkaline phosphatase, and gamma-glutamyl transferase should be obtained at baseline and periodically during therapy, especially when using high nafcillin doses; in patients with worsening hepatic
  • Renal tubular damage and interstitial nephritis are associated with the administration of nafcillin; manifestations of nephrotoxicity are hematuria, proteinuria, and acute kidney injury, and may be associated with rash, fever, and eosinophilia; the majority of cases resolve upon discontinuation of nafcillin; some patients may require dialysis treatment and may develop permanent renal damage
  • Clostridium difficile
    • C. difficile produces toxins A and B which contribute to the development of CDAD; hypertoxic producing strains of C. difficile cause increased morbidity and mortality, as these infections can be refractory to antimicrobial therapy and may require colectomy
    • CDAD must be considered in all patients who present with diarrhea following antibiotic use; careful medical history is necessary since CDAD has been reported to occur over two months after the administration of antibacterial agents
    • If clostridium difficile associated diarrhea (CDAD) is suspected or confirmed, may need to discontinue ongoing antibiotic use not directed against C. difficile
    • Appropriate fluid and electrolyte management, protein supplementation, antibiotic treatment of C. difficile, and surgical evaluation should be instituted as clinically indicated

Pregnancy and Lactation

  • May be acceptable during pregnancy.
  • Lactation
    • Excreted into breast milk; use caution 
Medscape. Nafcillin.

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