Pediatric Cephalexin (Keflex) Dosing
The FDA-approved standard pediatric dose of cephalexin is 25–50 mg/kg/day divided into doses given every 6–8 hours, with higher doses of 75–100 mg/kg/day (divided four times daily) specifically required for otitis media. 1
Standard Dosing by Indication
Mild-to-Moderate Infections
- Streptococcal pharyngitis, skin/soft tissue infections: 25–50 mg/kg/day divided every 12 hours (twice daily) or every 6–8 hours (3–4 times daily) 1
- For a 20 kg child, this translates to 250–500 mg per dose twice daily or 125–250 mg per dose four times daily 1
Severe Infections Requiring Higher Doses
- Otitis media: 75–100 mg/kg/day divided into 4 doses (every 6 hours) 1
- Bone and joint infections: Recent evidence supports 45 mg/kg/dose (maximum 1.5 g) three times daily as effective oral continuation therapy 2
- For severe infections, the FDA label states "dosage may be doubled" from the standard 25–50 mg/kg/day range 1
Infection-Specific Recommendations from Guidelines
Methicillin-Susceptible Staphylococcus aureus (MSSA) Infections
- Oral step-down therapy: Cephalexin 75–100 mg/kg/day divided into 3–4 doses is the preferred oral agent for MSSA skin and soft tissue infections 3
- Alternative to amoxicillin: Cephalexin 25 mg/kg/dose every 6–8 hours is appropriate for children ≥2 years with mild disease 3
Community-Acquired Pneumonia
- Oral therapy for MSSA pneumonia: Cephalexin 75–100 mg/kg/day in 3–4 divided doses 4
- This higher dose is necessary to achieve adequate drug exposure against MSSA with MICs up to 2 mg/L 5
Group A Streptococcal Pharyngitis
- Alternative to penicillin/amoxicillin: Cephalexin 20 mg/kg per dose twice daily (maximum 500 mg per dose) for 10 days in patients with non-anaphylactic penicillin allergy 6
- The 10-day duration is mandatory to prevent acute rheumatic fever 6
Practical Dosing Tables (FDA-Approved)
Four Times Daily Dosing (Every 6 Hours)
| Weight | 125 mg/5 mL suspension | 250 mg/5 mL suspension |
|---|---|---|
| 10 kg (22 lb) | 2.5–5 mL (½–1 tsp) | 1.25–2.5 mL (¼–½ tsp) |
| 20 kg (44 lb) | 5–10 mL (1–2 tsp) | 2.5–5 mL (½–1 tsp) |
| 40 kg (88 lb) | 10–20 mL (2–4 tsp) | 5–10 mL (1–2 tsp) |
Twice Daily Dosing (Every 12 Hours)
| Weight | 125 mg/5 mL suspension | 250 mg/5 mL suspension |
|---|---|---|
| 10 kg (22 lb) | 5–10 mL (1–2 tsp) | 2.5–5 mL (½–1 tsp) |
| 20 kg (44 lb) | 10–20 mL (2–4 tsp) | 5–10 mL (1–2 tsp) |
| 40 kg (88 lb) | 20–40 mL (4–8 tsp) | 10–20 mL (2–4 tsp) |
Evidence-Based Dosing Frequency Optimization
Twice Daily (BID) Dosing
- For MSSA with MIC ≤2 mg/L: 22–45 mg/kg/dose twice daily achieves pharmacodynamic targets (≥40% time above MIC) in ≥90% of children 7
- For MSSA with MIC 4 mg/L: 80 mg/kg/dose twice daily is required 7
Three Times Daily (TID) Dosing
- For MSSA with MIC ≤2 mg/L: 15–25 mg/kg/dose three times daily 7
- For bone/joint infections: 45 mg/kg/dose (maximum 1.5 g) three times daily achieved 99% cure rate with excellent tolerability 2
- For MSSA with MIC 4 mg/L: 45 mg/kg/dose three times daily 7
Age-Specific Considerations
Infants 7–60 Days Old
- For Enterobacterales: 25 mg/kg/dose every 6 hours achieves >90% target attainment 8
- For MSSA: 25 mg/kg/dose every 8 hours is sufficient 8
- Maturational changes in absorption and clearance require careful dose adjustment in this age group 8
Children >1 Year
- Standard FDA dosing applies: 25–50 mg/kg/day for most infections, 75–100 mg/kg/day for otitis media 1
Common Pitfalls to Avoid
- Do not cap doses at adult maximums (e.g., 500 mg four times daily) for children requiring weight-based dosing >15 kg; this leads to subtherapeutic exposure and treatment failure 9, 6
- Do not use cephalexin for penicillin-resistant Streptococcus pneumoniae (MIC >2 mg/L); high-dose amoxicillin (80–90 mg/kg/day) is required instead 4, 9
- Do not substitute cephalexin for amoxicillin in acute otitis media or sinusitis when β-lactamase-producing Haemophilus influenzae or Moraxella catarrhalis is suspected; use amoxicillin-clavulanate instead 9, 6
- Do not use four-times-daily dosing when twice- or three-times-daily regimens are equally effective; less frequent dosing improves adherence without compromising efficacy 2, 7
Duration of Therapy
- Streptococcal pharyngitis: 10 days mandatory to prevent rheumatic fever 6, 1
- Skin/soft tissue infections: 7–10 days 3
- Bone/joint infections: Extended courses (4–6 weeks total, including IV therapy) with oral step-down after clinical improvement 2
- Otitis media: 10 days for children <2 years; 5–7 days may be adequate for older children with uncomplicated disease 9
Monitoring and Adverse Effects
- Gastrointestinal symptoms occur in approximately 3% of children on high-dose regimens 2
- Mild neutropenia may develop in 9% of children on prolonged therapy but is typically asymptomatic and resolves without intervention 2
- Drug interaction with metformin: Cephalexin increases metformin exposure by 24–34%; dose adjustment and monitoring are required 1