What is the typical dosing regimen for cephalexin (cephalosporin antibiotic)?

Cephalexin Dosing

The standard adult dose of cephalexin is 250-500 mg orally every 6 hours (four times daily), with the usual adult dose being 250 mg every 6 hours for most infections, while pediatric dosing is 25-50 mg/kg/day divided into four doses. 1

Adult Dosing

• Standard dosing: 250 mg orally every 6 hours (four times daily) for most infections 1
• Alternative dosing: 500 mg every 12 hours (twice daily) may be used for streptococcal pharyngitis, skin and soft tissue infections, and uncomplicated cystitis in patients over 15 years of age 1
• Severe infections: 500 mg four times daily for skin and soft tissue infections caused by methicillin-susceptible Staphylococcus aureus (MSSA) 2
• Total daily dose range: 1-4 grams per day in divided doses 1
• Duration: Typically 7-10 days depending on clinical response and infection severity 2

Urinary Tract Infections

• Uncomplicated cystitis: 500 mg every 12 hours for 7-14 days 1
• Alternative regimen: 1 gram twice daily has demonstrated equivalent efficacy to four-times-daily dosing for chronic urinary tract infections 3
• Recent evidence: 500 mg twice daily is as effective as 500 mg four times daily for uncomplicated UTI in females 4

Pediatric Dosing

• Standard dose: 25-50 mg/kg/day divided into four doses (every 6 hours) 1
• Alternative twice-daily dosing: For streptococcal pharyngitis and skin/soft tissue infections in patients over 1 year of age, the total daily dose may be divided and administered every 12 hours 1
• Severe infections or MSSA: 75-100 mg/kg/day divided into 3-4 doses provides more aggressive coverage 2, 1
• Otitis media: 75-100 mg/kg/day in 4 divided doses is required 1
• Maximum daily dose: Should not exceed 4 grams per day 1

Evidence-Based Alternative Dosing for Children

• Twice-daily regimen: 22-45 mg/kg/dose for MSSA with MIC 1-2 mg/L; 80 mg/kg/dose for MIC 4 mg/L 5
• Three-times-daily regimen: 15-25 mg/kg/dose for MSSA with MIC 1-2 mg/L; 45 mg/kg/dose for MIC 4 mg/L 5
• Musculoskeletal infections: 25 mg/kg/dose (maximum 750 mg/dose) three times daily achieves pharmacodynamic targets 6

Critical Limitations and Contraindications

Cephalexin is completely ineffective against MRSA and should never be used if methicillin-resistant S. aureus is suspected or confirmed. 2

Spectrum Gaps

• No activity against: MRSA, Pseudomonas aeruginosa 2
• Poor activity against: Haemophilus influenzae, Moraxella catarrhalis 2
• Inadequate for bite wounds: Misses Pasteurella multocida (animal bites), Eikenella corrodens (human bites), and anaerobes 2

When NOT to Use Cephalexin

• Animal bite wounds: Use amoxicillin-clavulanate 875/125 mg twice daily instead 2
• Human bite wounds: Use amoxicillin-clavulanate 875/125 mg twice daily or ampicillin-sulbactam 1.5-3.0 g IV every 6 hours instead 2
• Severe penicillin allergy: Contraindicated in patients with history of anaphylaxis, angioedema, or urticaria to penicillins 2

Special Populations

Renal Impairment

• Creatinine clearance <30 mL/min: Dosage reduction required proportional to reduced renal function 7
• Excretion: 70-100% of dose appears in urine within 6-8 hours 7
• Urinary concentrations: 500-1000 mcg/mL following 250-500 mg oral doses 7

Streptococcal Infections

• β-hemolytic streptococcal infections: Therapeutic dosage must be administered for at least 10 days 1

Pharmacokinetic Considerations

• Absorption: Completely and rapidly absorbed in the upper intestine (not from stomach) 7
• Distribution: Rapid tissue distribution except CNS and aqueous humor 7
• Protein binding: Low 7
• Metabolism: No measurable destruction or metabolism in body fluids 7
• Half-life: Approximately 1.10 hours in children 6
• Pediatric considerations: Children may require higher doses per kilogram than adults due to greater body water turnover 7