Cephalexin (Keflex) 250 mg/5 mL Dosing Guidelines
Cephalexin 250 mg/5 mL suspension should be dosed at 25-50 mg/kg/day divided into four doses (every 6 hours) for most pediatric infections, with higher doses of 75-100 mg/kg/day required for serious infections like otitis media or methicillin-susceptible Staphylococcus aureus (MSSA) infections. 1, 2
Standard Pediatric Dosing by Weight
For the 250 mg/5 mL suspension formulation 1:
Mild to Moderate Infections (25-50 mg/kg/day):
- 10 kg (22 lb): 1/4 to 1/2 teaspoon (2.5-5 mL) four times daily 1
- 20 kg (44 lb): 1/2 to 1 teaspoon (5-10 mL) four times daily 1
- 40 kg (88 lb): 1 to 2 teaspoons (10-20 mL) four times daily 1
Twice-Daily Dosing Option (for streptococcal pharyngitis and skin infections in children >1 year) 1:
- 10 kg: 1/2 to 1 teaspoon (5-10 mL) twice daily 1
- 20 kg: 1 to 2 teaspoons (10-20 mL) twice daily 1
- 40 kg: 2 to 4 teaspoons (20-40 mL) twice daily 1
Infection-Specific Dosing
Severe Infections and Otitis Media:
- 75-100 mg/kg/day divided into 4 doses is required for adequate treatment 1, 2
- For a 20 kg child, this translates to approximately 1.5-2 teaspoons (15-20 mL) of the 250 mg/5 mL suspension four times daily 1
MSSA Infections:
- 75-100 mg/kg/day divided into 3-4 doses per IDSA guidelines 2, 3, 4
- This higher dosing ensures adequate tissue penetration for skin and soft tissue infections 3
Streptococcal Pharyngitis:
- Standard dosing (25-50 mg/kg/day) is adequate 1
- Must continue for at least 10 days to prevent rheumatic fever 1
Adult Dosing Reference
For adults, the standard dose is 500 mg (10 mL of 250 mg/5 mL suspension) every 6 hours, with 250 mg every 6 hours acceptable for mild infections 1, 4
Critical Microbiological Considerations
Cephalexin is ONLY effective against:
- Methicillin-susceptible Staphylococcus aureus (MSSA) 3, 4
- Streptococcus species 3
- Some gram-negative organisms (E. coli, Proteus mirabilis) 1
Cephalexin is INEFFECTIVE against:
- MRSA - switch to trimethoprim-sulfamethoxazole, doxycycline, or clindamycin 3, 4
- Pasteurella multocida (animal bites) 3
- Haemophilus influenzae (poor coverage) 4
- Moraxella catarrhalis 4
- Anaerobes 3
- Pseudomonas aeruginosa 4
Preparation and Storage Instructions
Reconstitution of 250 mg/5 mL suspension 1:
- Add 71 mL water to 100 mL bottle OR 140 mL water to 200 mL bottle 1
- Tap bottle to loosen powder, add water in 2 portions, shake well after each addition 1
- Store in refrigerator after mixing 1
- Discard after 14 days 1
Duration of Therapy
- Streptococcal infections: Minimum 10 days 1
- Skin and soft tissue infections: 7-10 days 4
- Uncomplicated cystitis: 7-14 days 1
Common Pitfalls to Avoid
Underdosing severe infections: Many clinicians use 25-50 mg/kg/day for all infections, but otitis media and MSSA infections require 75-100 mg/kg/day for adequate response 1, 2
Using cephalexin for MRSA: If community-acquired MRSA is suspected (purulent infections, abscesses), cephalexin will fail - use clindamycin or trimethoprim-sulfamethoxazole instead 3, 4
Inadequate treatment duration for streptococcal pharyngitis: Stopping before 10 days risks rheumatic fever 1
Penicillin allergy cross-reactivity: Patients with immediate hypersensitivity reactions (anaphylaxis, angioedema, urticaria) to penicillin should avoid cephalexin 3