What is the recommended dosage of Keflex (cephalexin) for treating bacterial infections?

Cephalexin (Keflex) Dosing Recommendations

Adult Dosing

For most bacterial infections in adults, cephalexin should be dosed at 500 mg four times daily (every 6 hours), which is the standard recommendation from the Infectious Diseases Society of America for skin and soft tissue infections caused by methicillin-susceptible Staphylococcus aureus (MSSA). 1, 2

Standard Adult Regimens:

• Most infections: 500 mg every 6 hours (four times daily) 1, 3
• Mild to moderate infections: 250-500 mg every 6 hours 1, 3
• Streptococcal pharyngitis, uncomplicated skin infections, and uncomplicated cystitis: 500 mg every 12 hours (twice daily) is acceptable 3
• Severe infections: May require up to 4 grams daily in divided doses; if more than 4 grams daily is needed, consider parenteral cephalosporins 3

Duration:

• Typical course: 7-10 days for most infections 1
• Uncomplicated cystitis: 7-14 days 3
• Beta-hemolytic streptococcal infections: Minimum 10 days 3

Pediatric Dosing

For children with MSSA infections, the American Academy of Pediatrics recommends 75-100 mg/kg/day divided into 3-4 doses. 1, 2

Standard Pediatric Regimens:

• Mild to moderate infections: 25-50 mg/kg/day divided into 4 doses 1, 3
• MSSA infections: 75-100 mg/kg/day divided into 3-4 doses 1, 2
• Otitis media: 75-100 mg/kg/day in 4 divided doses 3
• Streptococcal pharyngitis and skin infections (>1 year): Total daily dose may be divided every 12 hours 3
• Severe infections: Dosage may be doubled 3

Practical Dosing by Weight:

For 25-50 mg/kg/day regimen (four times daily) 3:

• 10 kg child: 1/2 to 1 teaspoon of 125 mg/5 mL suspension four times daily
• 20 kg child: 1 to 2 teaspoons of 125 mg/5 mL suspension four times daily
• 40 kg child: 2 to 4 teaspoons of 125 mg/5 mL suspension four times daily

Infection-Specific Considerations

Skin and Soft Tissue Infections:

• Adults: 500 mg four times daily 1, 2, 4
• Children with mild infections: 25-50 mg/kg/day in 4 divided doses 1
• Children with MSSA: 75-100 mg/kg/day in 3-4 divided doses 1, 2

Mastitis:

• Lactating women: 500 mg every 6 hours (four times daily) provides effective coverage against Staphylococcus aureus and achieves therapeutic levels in breast milk 1

Urinary Tract Infections:

• Adults: 500 mg twice daily is effective for uncomplicated cystitis, with recent evidence supporting twice or thrice daily dosing as fluoroquinolone-sparing alternatives 5

Critical Pitfalls and Limitations

When NOT to Use Cephalexin:

• MRSA infections: Cephalexin is completely ineffective against methicillin-resistant S. aureus; use trimethoprim-sulfamethoxazole, doxycycline, or clindamycin instead 1, 2, 4
• Haemophilus influenzae infections: Poor activity makes it inappropriate for sinusitis and many respiratory infections 1
• Moraxella catarrhalis: Inadequate activity against beta-lactamase-producing strains 1
• Pseudomonas aeruginosa: Completely ineffective 1
• Pasteurella multocida: Limited activity, important for animal bite wounds 2, 4
• Anaerobes: Limited coverage 2
• Endocarditis prophylaxis: Not recommended due to 96% resistance rates among viridans group streptococci 1

Allergy Considerations:

• Severe penicillin allergy: Avoid cephalexin in patients with history of anaphylaxis, angioedema, or urticaria to penicillins due to cross-reactivity 1, 4
• Non-immediate penicillin reactions: Cephalexin remains an appropriate alternative 2, 4

Renal Dosing Adjustments

Patients with creatinine clearance less than 30 mL/min require dose reduction proportional to their reduced renal function. 6

• Cephalexin is 70-100% renally excreted unchanged within 6-8 hours 6
• Dose adjustment should be based on creatinine clearance or serum creatinine 6

Pharmacokinetic Advantages

• Rapid and complete absorption: Absorbed high in the intestinal tract without disturbing lower bowel flora 6
• High urinary concentrations: 500-1000 mcg/mL following 250-500 mg oral doses, far exceeding MIC for typical uropathogens 6
• Low protein binding: Minimal tissue cell penetration accounts for low side effect profile 6
• No metabolism: Excreted unchanged, simplifying drug interactions 6