What is the recommended dosage of Keflex (cephalexin) for adults and children for different infection severities, including adjustments for renal impairment?

Keflex Dosage

For adults, Keflex is dosed at 250-500 mg every 6 hours (or 500 mg every 12 hours for uncomplicated infections), with a maximum of 4 g daily; for children, the standard dose is 25-50 mg/kg/day divided into 4 doses, with higher doses (75-100 mg/kg/day) for severe infections like otitis media. 1

Adult Dosing

Standard infections:

• 250 mg every 6 hours for mild infections
• 500 mg every 12 hours for streptococcal pharyngitis, uncomplicated skin infections, and uncomplicated cystitis (patients >15 years)
• 500 mg every 6 hours for more severe infections or less susceptible organisms
• Maximum: 4 g daily (if higher doses needed, consider parenteral cephalosporins) 1

Specific indications from guidelines:

• Skin/soft tissue infections (MSSA): 500 mg four times daily 2, 3, 4
• Methicillin-susceptible S. aureus pneumonia (children): 75-100 mg/kg/day in 3-4 doses 5, 6
• UTI duration: 7-14 days 1

Pediatric Dosing

Standard dose: 25-50 mg/kg/day divided every 6 hours (four times daily) 1

Condition-specific dosing:

• Streptococcal pharyngitis and skin infections: May divide total daily dose every 12 hours (twice daily) for patients >1 year 1
• Otitis media: 75-100 mg/kg/day in 4 divided doses 1
• Severe infections: Double the standard dose 1
• UTI (infants >28 days): 50-100 mg/kg/day in 4 doses 7
• Bone and joint infections: Recent evidence supports 45 mg/kg/dose (maximum 1.5 g) three times daily as effective and well-tolerated 8

Important: For β-hemolytic streptococcal infections, continue therapy for at least 10 days 1

Renal Impairment Adjustments

Dosing must be reduced when creatinine clearance <30 mL/min 9, 10. The elimination rate correlates directly with creatinine clearance (Ke = 0.0766 + 0.0060 × CrCl) 9.

Specific adjustments:

• Normal renal function: Serum half-life ~1 hour; 64% urinary recovery in 6 hours 9
• Anephric patients: Serum half-life increases to ~8.5 hours 9
• Hemodialysis: Removes 58% of serum cephalexin over 6 hours 11
• Practical approach: Reduce dose proportionally to reduced creatinine clearance 10

Clinical Considerations

Absorption and timing:

• Cephalexin is completely absorbed from the upper intestine (not stomach) 10
• Can be given without regard to meals, though food may improve GI tolerability 1
• Peak levels occur at 2 hours in normal patients 9

Common pitfalls:

• Children have greater body water turnover and may require higher per-kilogram doses than adults 10
• Delayed absorption can occur in anephric patients (peak levels at 6-12 hours in some cases) 11
• The standard four-times-daily dosing creates adherence challenges; twice-daily dosing may be reasonable for UTIs 12 and three-times-daily for bone/joint infections 8

Safety profile:

• Low protein binding and no metabolism in the body 10
• Does not penetrate host cells, contributing to low side effect incidence 10
• Minimal disruption of lower bowel flora due to upper intestinal absorption 10
• Most common adverse effect is diarrhea; neutropenia can occur but is usually mild 8