What is the appropriate oral dose of Keflex (cephalexin) for treating acute bacterial paronychia in an otherwise healthy adult?

Cephalexin Dosing for Acute Bacterial Paronychia

For acute bacterial paronychia in otherwise healthy adults, prescribe cephalexin 500 mg orally four times daily (every 6 hours) for 7-10 days. 1, 2, 3

Standard Adult Dosing Regimen

• The recommended dose is 500 mg orally every 6 hours (four times daily) for skin and soft tissue infections caused by methicillin-susceptible Staphylococcus aureus (MSSA) and Streptococcus species. 1, 2, 3

• The FDA label specifies that for more severe infections or those caused by less susceptible organisms, larger doses may be needed, with the adult dosage ranging from 1 to 4 grams daily in divided doses. 3

• The four-times-daily regimen is essential because cephalexin's short half-life requires dosing every six hours to maintain effective tissue concentrations—three-times-daily dosing is inadequate and may lead to subtherapeutic levels. 2

Duration of Therapy

• Treatment should continue for 7-10 days depending on clinical response. 1, 2, 4

• Reassess the patient after 2 weeks; if the infection worsens or does not improve, consider obtaining bacterial cultures and escalating therapy. 5

Critical Pitfalls: When Cephalexin Is NOT Appropriate

MRSA Suspected or Confirmed

• Cephalexin is completely ineffective against methicillin-resistant Staphylococcus aureus (MRSA). 1, 2, 4

• If MRSA is suspected (e.g., patient has failed initial therapy, has systemic signs of infection, or has risk factors for MRSA), switch immediately to MRSA-active agents: 1, 2

  • Trimethoprim-sulfamethoxazole: 1-2 double-strength tablets twice daily
  • Clindamycin: 300-450 mg every 6 hours
  • Doxycycline: 100 mg twice daily

Severe Penicillin Allergy

• Cephalexin may be used in patients with non-severe penicillin allergy (e.g., delayed rash). 1, 4

• It is contraindicated in patients with a history of anaphylaxis, angioedema, or urticaria to penicillins due to potential cross-reactivity with cephalosporins. 1

Microbiological Coverage and Limitations

• Cephalexin provides excellent activity against methicillin-susceptible Staphylococcus aureus and Streptococcus species, the most common pathogens in acute bacterial paronychia. 1, 2, 4

• Cephalexin has poor or no activity against:

  • MRSA 1, 2, 4
  • Pasteurella multocida (relevant for animal bite-related paronychia) 1, 4
  • Most anaerobes 1, 2
  • Pseudomonas aeruginosa 2

Clinical Context for Paronychia

• Secondary bacterial superinfection occurs in up to 25% of paronychia cases, with both gram-positive and gram-negative organisms implicated. 5

• If infection is suspected (Grade 2 or higher paronychia with pain, discharge, or nail plate separation), obtain bacterial, viral, and fungal cultures before initiating or escalating antibiotic therapy. 5

• For Grade 1 paronychia (nail fold edema or erythema without pain), topical therapy with povidone iodine 2% or topical antibiotics/corticosteroids may suffice without systemic antibiotics. 5

Monitoring and Follow-Up

• Reassess after 2 weeks; if the infection has not improved within 5-7 days, consider treatment failure and broaden antimicrobial coverage or obtain cultures. 5, 4

• If systemic signs of infection develop (fever, spreading erythema, lymphangitis), consider parenteral antibiotics or surgical intervention. 5