What is the recommended cephalexin (Keflex) dosage for an acute dental infection in adults, children, and patients with renal impairment, and what are alternative antibiotics for cephalosporin‑ or penicillin‑allergic patients?

Cephalexin (Keflex) Dosing for Dental Infections

For acute dental infections in adults with normal renal function, prescribe cephalexin 500 mg orally every 6 hours (four times daily) for 7–10 days to achieve adequate tissue concentrations against methicillin-susceptible Staphylococcus aureus and streptococcal species. 1

Adult Dosing

• Standard regimen: 500 mg orally every 6 hours (QID) for 7–10 days is recommended by the Infectious Diseases Society of America for dental and oral infections. 1
• The four-times-daily schedule is critical because cephalexin's short half-life requires dosing every 6 hours to maintain therapeutic tissue levels; three-times-daily dosing is inadequate. 1
• For mild dental infections, a lower dose of 250–500 mg every 6 hours may be considered, though 500 mg QID is preferred for optimal penetration. 1, 2
• Cephalexin achieves good distribution into oral tissues, with tissue levels ranging from 1.30 to 18.0 mcg/g after a 1 g dose. 3

Pediatric Dosing

• For children with mild to moderate dental infections: 25–50 mg/kg/day divided into 4 doses (approximately every 6 hours). 1
• For more severe infections or confirmed MSSA: 75–100 mg/kg/day divided into 3–4 doses. 1
• The liquid suspension formulation makes cephalexin practical for pediatric administration. 1
• Children may require higher doses per kilogram than adults due to greater body water turnover. 4

Renal Impairment Dosing

• Patients with creatinine clearance < 30 mL/min require dose reduction proportional to their reduced renal function. 4
• Cephalexin is 70–100% renally excreted within 6–8 hours, necessitating adjustment in renal dysfunction. 4
• Determine creatinine clearance or serum creatinine to guide dose reduction. 4

Alternatives for Penicillin/Cephalosporin Allergy

Non-Severe Penicillin Allergy (Delayed Rash)

• Cephalexin may still be used cautiously in patients with non-severe, non-IgE-mediated penicillin reactions. 1, 2

Severe Penicillin Allergy (Anaphylaxis, Angioedema, Urticaria)

• Cephalexin is contraindicated due to cross-reactivity risk. 5, 1
• Alternative: Clindamycin 300–450 mg orally every 6 hours (QID) for adults. 6
• Alternative: Azithromycin 500 mg on day 1, then 250 mg daily for 4 days (15 mg/kg single dose for pediatric endocarditis prophylaxis). 5
• For children allergic to penicillin: Clindamycin 20 mg/kg orally as a single prophylactic dose or 30–40 mg/kg/day divided TID for treatment. 5, 1

Critical Clinical Considerations

When Cephalexin Is Appropriate

• Cephalexin provides excellent coverage against methicillin-susceptible S. aureus (MSSA) and streptococcal species, the most common dental pathogens. 1, 2
• It is effective for periapical abscesses, acute periodontitis, and post-extraction infections when MSSA is suspected. 1, 7

When Cephalexin Is NOT Appropriate

• MRSA suspected or confirmed: Switch immediately to trimethoprim-sulfamethoxazole (1–2 double-strength tablets BID in adults; 8–12 mg/kg/day divided BID in children) or clindamycin. 1
• Anaerobic infections or deep abscesses: Cephalexin has limited anaerobic coverage; consider clindamycin or amoxicillin-clavulanate instead. 1
• Haemophilus influenzae or Moraxella catarrhalis suspected: Cephalexin has poor activity against these organisms and should not be used for sinusitis or respiratory infections. 1, 8
• Endocarditis prophylaxis: The American Heart Association recommends against cephalexin due to 96% resistance rates among viridans group streptococci; use amoxicillin 2 g (50 mg/kg in children) as first-line instead. 5, 1

Duration and Monitoring

• Standard duration: 7–10 days depending on clinical response. 1, 2
• If no improvement within 5 days, extend treatment or consider alternative diagnosis/pathogen (e.g., MRSA, anaerobes). 2
• Monitor for antibiotic-associated diarrhea and Clostridioides difficile infection, particularly with prolonged courses. 1

Common Pitfalls to Avoid

• Do not use three-times-daily dosing: This results in subtherapeutic levels due to cephalexin's short half-life. 1
• Do not extend prophylactic antibiotics beyond 24 hours post-operatively for routine dental procedures; this increases resistance and adverse effects without benefit. 1
• Do not prescribe cephalexin for patients with severe penicillin allergy history (anaphylaxis, angioedema, urticaria) due to 1–10% cross-reactivity risk. 5, 1
• Do not use cephalexin as monotherapy for bite wounds due to poor coverage of Pasteurella multocida (animal bites) and Eikenella corrodens (human bites). 1