What is the recommended dosage of cefalexin (cephalexin) for a 3-month-old infant?

Cephalexin Dosing for a 3-Month-Old Infant

For a 3-month-old infant, cephalexin should be dosed at 25-50 mg/kg/day divided into 4 doses (every 6 hours), with the specific dose depending on the infection type and severity. 1

Standard Dosing Framework

The FDA-approved pediatric dosing for cephalexin is 25-50 mg/kg/day in divided doses, which serves as the foundation for all indications. 1 This translates to approximately 6.25-12.5 mg/kg per dose when given every 6 hours (four times daily). 1

Indication-Specific Dosing

For Methicillin-Susceptible Staphylococcus aureus (MSSA) Infections

• Use 75-100 mg/kg/day divided into 3 or 4 doses for MSSA infections including community-acquired pneumonia. 2
• This higher dosing (approximately 18.75-25 mg/kg per dose every 6 hours) ensures adequate coverage for more serious staphylococcal infections. 2

For Urinary Tract Infections

• For infants 29-60 days old (which includes your 3-month-old patient), use 50-100 mg/kg/day divided into 4 doses as oral step-down therapy after initial parenteral treatment. 2
• This equates to 12.5-25 mg/kg per dose every 6 hours. 2

For Otitis Media

• Use 75-100 mg/kg/day in 4 divided doses for adequate coverage. 1
• Clinical studies specifically demonstrate that this higher dosing range is required for otitis media treatment. 1

Critical Age-Related Considerations

Important caveat: For serious infections in infants under 28 days old or those requiring hospitalization (meningitis, bacteremia without identified source), parenteral antibiotics are preferred over oral cephalexin. 2 At 3 months of age, your patient is beyond this critical threshold, making oral cephalexin appropriate for suitable infections. 2

Practical Administration

• Suspension formulation: Use either 125 mg/5 mL or 250 mg/5 mL suspension based on the calculated dose. 1
• Dosing frequency: The standard is every 6 hours (four times daily), though for streptococcal pharyngitis and skin infections, twice-daily dosing may be acceptable. 1
• Duration: For β-hemolytic streptococcal infections, continue for at least 10 days. 1

Common Pitfalls to Avoid

• Do not use cephalexin as first-line therapy for serious infections requiring hospitalization in young infants—parenteral ampicillin plus ceftazidime or gentamicin is the standard of care. 2
• Do not underdose: The 25 mg/kg/day lower end is only appropriate for mild infections; most infections require 50-100 mg/kg/day. 2, 1
• Refrigeration required: After mixing, the suspension must be refrigerated and discarded after 14 days. 1