Oral Cephalexin Dosing for 90-lb Patient
For a 90-lb (41-kg) patient, administer oral cephalexin 1000-1640 mg per day divided into 3-4 doses, which translates to 250-500 mg every 6-8 hours depending on infection severity. 1
Standard Dosing Calculation
The FDA-approved pediatric dosing for cephalexin is 25-50 mg/kg/day in divided doses 1. For a 41-kg patient:
- Standard dose: 25 mg/kg/day = 1025 mg/day
- Higher dose for severe infections: 50 mg/kg/day = 2050 mg/day
Practical Liquid Dosing Regimens
For Mild-to-Moderate Infections
Using the 250 mg/5 mL suspension 1:
- 1-2 teaspoons (250-500 mg) every 6 hours (4 times daily)
- Total daily dose: 1000-2000 mg
For Severe Infections or Less Susceptible Organisms
- Double the standard dose as indicated by FDA labeling 1
- Maximum consideration: up to 4 grams daily (though parenteral therapy should be considered if exceeding this) 1
Specific Clinical Context Dosing
For methicillin-susceptible Staphylococcus aureus infections (the primary indication where cephalexin appears in guidelines):
- 75-100 mg/kg/day divided into 3-4 doses 2, 3
- For 41 kg: 3075-4100 mg/day divided into 3-4 doses
- Practical: 750-1000 mg every 6-8 hours
Dosing Frequency Options
The FDA label provides flexibility 1:
- Every 6 hours (QID): Standard for most infections
- Every 12 hours (BID): Acceptable for streptococcal pharyngitis, skin/soft tissue infections, and uncomplicated cystitis in patients >15 years
Recent evidence supports three times daily dosing at 45 mg/kg/dose (maximum 1.5 g) for bone and joint infections, showing 99% cure rates with good tolerability 4. For a 41-kg patient, this would be 1500 mg three times daily.
Important Caveats
- Suspension stability: After reconstitution, refrigerate and discard after 14 days 1
- Severe infections: May require doubling the dose 1
- Treatment duration: Minimum 10 days for beta-hemolytic streptococcal infections 1
- Otitis media: Requires higher dosing at 75-100 mg/kg/day in 4 divided doses 1
The most practical approach for a 90-lb patient with typical bacterial infection is 500 mg (2 teaspoons of 250 mg/5 mL suspension) every 6-8 hours, adjusting upward to 750-1000 mg per dose for severe infections.