Cephalexin Dosing for an 11-Year-Old Weighing 160 lbs (72 kg)
For this 11-year-old child weighing 72 kg, prescribe cephalexin 75–100 mg/kg/day divided into 3 or 4 doses, which translates to 1,350–1,800 mg three times daily (TID) or 1,000–1,350 mg four times daily (QID), with a maximum single dose of 1,500 mg.
Dosing Framework
The FDA-approved pediatric dosing for cephalexin is 25–50 mg/kg/day in divided doses for most infections 1. However, the specific indication significantly impacts the appropriate dose:
Standard Dosing (Most Common Infections)
- 25–50 mg/kg/day divided every 6–12 hours
- For this 72 kg child: 450–900 mg TID or 340–675 mg QID
- Maximum daily dose: 4 grams 1
Higher-Intensity Dosing (Serious Infections)
For methicillin-susceptible Staphylococcus aureus (MSSA) infections, including skin/soft tissue infections and bone/joint infections, significantly higher doses are required:
- 75–100 mg/kg/day divided into 3 or 4 doses 2, 3, 2, 3, 2
- For this 72 kg child: 1,350–1,800 mg TID or 1,000–1,350 mg QID
- Recent evidence supports 45 mg/kg/dose (maximum 1,500 mg) TID for bone/joint infections 4
Otitis Media Specific Dosing
- 75–100 mg/kg/day in 4 divided doses is required for otitis media 1
- For this 72 kg child: 1,000–1,350 mg QID
Practical Dosing Recommendations
Without knowing the specific indication, the safest approach for this child is:
For mild-moderate infections (pharyngitis, uncomplicated skin infections):
- 500 mg every 12 hours (BID) 1
- This is FDA-approved for streptococcal pharyngitis and skin/skin structure infections
For more serious infections (suspected MSSA, complicated skin infections, bone/joint involvement):
- 1,500 mg TID (45 mg/kg/dose) 4
- This achieves optimal pharmacodynamic targets against MSSA
For otitis media:
- 1,350 mg QID (approximately 75 mg/kg/day) 1
Key Clinical Considerations
Weight-Based Dosing Ceiling
This child weighs 72 kg, which exceeds typical pediatric weight ranges used in most studies. The FDA label states that doses greater than 4 g/day should prompt consideration of parenteral cephalosporins 1. At 75–100 mg/kg/day, this child would receive 5.4–7.2 g/day, which exceeds this threshold.
Therefore, cap the total daily dose at 4 grams (e.g., 1,000 mg QID or 1,333 mg TID), unless treating a serious MSSA infection where higher doses up to 4.5 g/day (1,500 mg TID) are supported by recent evidence 4.
Dosing Frequency Trade-offs
- QID dosing (every 6 hours) is FDA-standard but challenging for adherence 1
- TID dosing (every 8 hours) improves compliance and is supported by pharmacokinetic modeling for MSSA infections 4, 5
- BID dosing (every 12 hours) is FDA-approved only for specific indications (streptococcal pharyngitis, uncomplicated skin infections, uncomplicated cystitis in patients >15 years) 1
Pharmacodynamic Targets
Cephalexin achieves efficacy when free drug concentrations exceed the minimum inhibitory concentration (MIC) for ≥40% of the dosing interval 6, 5, 7. For MSSA with MIC ≤2 mg/L, standard dosing suffices. For MIC of 4 mg/L, higher doses (45 mg/kg TID or 80 mg/kg BID) are required 5.
Common Pitfalls
Underdosing serious infections: Using 25–50 mg/kg/day for MSSA infections will likely fail. The IDSA/PIDS guidelines explicitly recommend 75–100 mg/kg/day for oral step-down therapy of MSSA pneumonia and other serious infections 2, 3.
Exceeding maximum doses without justification: While 1,500 mg TID (4.5 g/day) is supported for bone/joint infections 4, this exceeds the FDA's 4 g/day recommendation for general use 1.
Ignoring adherence: QID dosing has poor real-world compliance. Consider TID dosing when appropriate 4, 5.
Duration errors: For β-hemolytic streptococcal infections, treat for at least 10 days 1.