What is the appropriate cephalexin (Keflex) dose for a child weighing 50 lb (≈22.7 kg)?

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Last updated: October 15, 2026 • View editorial policy

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Cephalexin Dosing for a 50 lb (22.7 kg) Child

For a 50 lb child, administer cephalexin 25-50 mg/kg/day divided into 3-4 doses, which translates to 570-1140 mg/day total, or approximately 190-380 mg per dose if given three times daily, or 140-285 mg per dose if given four times daily.

Standard Dosing Framework

The FDA-approved dosing for cephalexin in pediatric patients is 25-50 mg/kg/day in divided doses 1. For this 22.7 kg child:

  • Lower range (25 mg/kg/day): 567.5 mg/day total
  • Higher range (50 mg/kg/day): 1135 mg/day total

Dosing by Indication

For Skin and Soft Tissue Infections (SSTI)

Dose: 75-100 mg/kg/day divided into 3-4 doses 2, 3

For methicillin-susceptible Staphylococcus aureus (MSSA) infections, the IDSA/PIDS guidelines recommend oral cephalexin at 75-100 mg/kg/day. For this 22.7 kg child:

  • Total daily dose: 1700-2270 mg/day
  • If divided TID: 570-760 mg per dose
  • If divided QID: 425-570 mg per dose

The IDSA 2014 skin and soft tissue infection guidelines also support cephalexin at 25-50 mg/kg/day in 4 divided doses for MSSA 4.

For Streptococcal Pharyngitis

Dose: 25-50 mg/kg/day, may be divided every 12 hours 1

For this indication, the total daily dose may be split into twice-daily dosing for convenience. For the 22.7 kg child:

  • Total daily dose: 570-1140 mg/day
  • If divided BID: 285-570 mg per dose

For Otitis Media

Dose: 75-100 mg/kg/day in 4 divided doses 1

Clinical studies demonstrate this higher dosing is required for otitis media:

  • Total daily dose: 1700-2270 mg/day
  • Divided QID: 425-570 mg per dose

Practical Dosing Considerations

Frequency Options

Recent pharmacokinetic studies support less frequent dosing to improve adherence:

Three times daily (TID) dosing: A 2025 study demonstrated that 45 mg/kg/dose (maximum 1.5 g) TID achieves appropriate pharmacodynamic targets and is effective for bone and joint infections 5. For this 22.7 kg child, this would be approximately 1020 mg per dose TID.

Twice daily (BID) dosing: For MSSA with MIC ≤2 mg/L, doses of 22-45 mg/kg BID achieve adequate targets 6. This translates to 500-1020 mg per dose BID for this child.

Common Pitfalls

  1. Underdosing for serious infections: The standard 25-50 mg/kg/day is appropriate for uncomplicated infections, but serious MSSA infections (osteomyelitis, septic arthritis) require 75-100 mg/kg/day 2, 3.

  2. Weight-based maximum confusion: Patients over 20 kg are at highest risk for dosing errors, particularly overdosing when prescribers apply adult maximum doses inappropriately 7.

  3. Frequency adherence: Four times daily dosing has poor adherence. Consider TID or BID regimens when appropriate for the indication 6, 8.

Duration of Therapy

  • Streptococcal infections: Minimum 10 days 1
  • Uncomplicated cystitis: 7-14 days 1
  • Skin infections: 7 days depending on clinical response 4

Algorithm for Dose Selection

  1. Identify the infection type:

    • Simple SSTI or pharyngitis → 25-50 mg/kg/day
    • Serious MSSA infection (bone/joint) → 75-100 mg/kg/day
    • Otitis media → 75-100 mg/kg/day

  2. Calculate total daily dose based on 22.7 kg weight

  3. Choose frequency:

    • QID for standard dosing or otitis media
    • TID for improved adherence (use higher per-dose amount)
    • BID only for pharyngitis or mild SSTI

  4. Round to practical suspension volumes based on available concentrations (typically 125 mg/5 mL or 250 mg/5 mL)

References

7
Research

Evaluating pediatric antimicrobial dosing of β-lactam antibiotics for upper respiratory tract infections in emergency and primary care settings.

American journal of health-system pharmacy : AJHP : official journal of the American Society of Health-System Pharmacists, 2025

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