What is the appropriate cephalexin (Keflex) dose for a 12‑year‑old child weighing approximately 40‑50 kg?

Cephalexin (Keflex) Dosing for a 12-Year-Old

For a 12-year-old child weighing 40–50 kg, the standard cephalexin dose is 25–50 mg/kg/day divided into 3–4 doses (every 6–8 hours), which translates to approximately 1,000–2,500 mg total daily dose, typically given as 250–500 mg every 6 hours or 500–750 mg every 8–12 hours depending on infection severity. 1

Weight-Based Dosing Algorithm

Standard Dosing for Most Infections

• The FDA-approved pediatric dosing is 25–50 mg/kg/day divided into doses every 6–8 hours. 1
• For a 40 kg child: this equals 1,000–2,000 mg/day (250–500 mg every 6 hours, or 333–667 mg every 8 hours). 1
• For a 50 kg child: this equals 1,250–2,500 mg/day (312–625 mg every 6 hours, or 417–833 mg every 8 hours). 1

Practical Dosing Regimens

For mild-to-moderate infections (skin/soft tissue, uncomplicated UTI, pharyngitis):

• 250–500 mg every 6 hours (four times daily), or
• 500 mg every 12 hours (twice daily) for streptococcal pharyngitis and uncomplicated skin infections. 1

For more severe infections or less susceptible organisms:

• 500 mg every 6 hours (2,000 mg/day total). 1
• The FDA label states that "for more severe infections or those caused by less susceptible organisms, larger doses may be needed." 1

Otitis Media (Specific Indication)

• Clinical studies demonstrate that 75–100 mg/kg/day in 4 divided doses is required for otitis media. 1
• For a 40–50 kg child, this translates to 750–1,250 mg every 6 hours (3,000–5,000 mg/day total). 1

Infection-Specific Considerations

Streptococcal Pharyngitis

• Minimum 10-day course is mandatory to prevent rheumatic fever complications. 1
• Either 250–500 mg every 6 hours or 500 mg every 12 hours is acceptable. 1

Skin and Soft Tissue Infections (Staphylococcus aureus)

• For methicillin-susceptible S. aureus (MSSA), cephalexin is first-line therapy. 2, 3
• The IDSA/PIDS guidelines recommend 75–100 mg/kg/day in 3–4 divided doses for oral step-down therapy. 2
• Recent pharmacokinetic studies support less frequent dosing: 22–45 mg/kg/dose three times daily achieves adequate drug levels for MSSA with MIC ≤2 mg/L. 3, 4

Urinary Tract Infections

• Cephalexin achieves urinary concentrations of 500–1,000 mcg/mL following 250–500 mg oral doses, far exceeding MICs for common uropathogens. 5, 6
• Therapy should continue for 7–14 days for cystitis. 1

Alternative Dosing Frequencies to Improve Adherence

Three-Times-Daily Dosing

• Recent evidence supports 45 mg/kg/dose (maximum 1.5 g) three times daily for bone/joint infections, with 99% cure rates and good tolerability. 4
• For a 40–50 kg child: 1,500 mg three times daily (at the maximum cap). 4
• This regimen reduces pill burden compared to four-times-daily dosing while maintaining efficacy. 3, 4

Twice-Daily Dosing

• Pharmacokinetic modeling suggests 22–45 mg/kg/dose twice daily achieves targets for MSSA with MIC ≤2 mg/L. 3
• For streptococcal pharyngitis and uncomplicated skin infections, 500 mg every 12 hours is FDA-approved for patients >15 years. 1
• A 12-year-old at 40–50 kg approaches adult weight thresholds and may use 500–750 mg twice daily for appropriate indications. 1

Maximum Dose Limitations

• Daily doses >4 g require consideration of parenteral cephalosporins rather than escalating oral cephalexin further. 1
• For severe infections requiring aggressive dosing, ceftriaxone 50–100 mg/kg/day IV (maximum 2–4 g/day) is preferred over high-dose oral cephalexin. 2, 7

Common Pitfalls and Caveats

Do Not Underdose Serious Infections

• The lower end of the dosing range (25 mg/kg/day) is insufficient for bone/joint infections, severe skin infections, or otitis media—always use 75–100 mg/kg/day for these indications. 1, 4

Cephalexin Has Limited Spectrum

• Cephalexin is inactive against Borrelia burgdorferi (Lyme disease), atypical respiratory pathogens (Mycoplasma, Chlamydia), and many gram-negative organisms beyond E. coli and Proteus. 2, 8
• For community-acquired pneumonia, amoxicillin 90 mg/kg/day or ceftriaxone is preferred over cephalexin because cephalexin has poor activity against Streptococcus pneumoniae and Haemophilus influenzae. 2, 9

Refrigeration and Stability

• Oral suspension must be refrigerated and discarded after 14 days. 1

Renal Dosing Adjustment

• Patients with creatinine clearance <30 mL/min require dose reduction proportional to renal function. 5

Clinical Monitoring

• Clinical improvement should occur within 48–72 hours; lack of response warrants reassessment for resistant organisms, abscess formation, or alternative diagnoses. 9
• For streptococcal infections, complete the full 10-day course even if symptoms resolve earlier. 1