What is the dosing for cephalexin (antibiotic) in adults and pediatric patients with varying levels of infection severity and impaired renal function?

Cephalexin Dosing Guidelines

For adults with normal renal function, cephalexin should be dosed at 500 mg four times daily (or 250-500 mg every 6 hours) for most infections, while pediatric patients should receive 75-100 mg/kg/day divided into 3-4 doses. 1

Adult Dosing

Standard Dosing for Adults

• 500 mg four times daily (every 6 hours) for most infections 1
• 250-500 mg four times daily for mild to moderate infections 1
• For skin and soft tissue infections, particularly MSSA infections: 500 mg four times daily 1

Alternative Dosing Schedules

• Recent evidence suggests that 500 mg twice daily may be as effective as four times daily dosing for uncomplicated urinary tract infections in women 2
• For recurrent skin infections: 500 mg every 6 hours 1

Dosing in Renal Impairment

• Patients with creatinine clearance less than 30 ml/min require dose reduction proportional to the reduced renal function 3
• Dose should be adjusted based on creatinine clearance or serum creatinine levels 3, 4

Pediatric Dosing

Standard Pediatric Dosing

• 75-100 mg/kg/day divided into 3-4 doses for methicillin-susceptible Staphylococcus aureus (MSSA) infections 1
• 25-50 mg/kg/day in 4 divided doses for mild to moderate infections 1

Alternative Pediatric Dosing Schedules

• Recent pharmacokinetic studies support twice-daily (BID) dosing at 22-45 mg/kg for MSSA with MICs of 1-2 mg/L and 80 mg/kg for MICs of 4 mg/L 5
• Three times daily (TID) dosing at 15-25 mg/kg for MSSA with MICs of 1-2 mg/L and 45 mg/kg for MICs of 4 mg/L 5

Infection-Specific Dosing

Skin and Soft Tissue Infections

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

Urinary Tract Infections

• Adults: 500 mg four times daily traditionally, though 500 mg twice daily may be equally effective for uncomplicated UTIs in women 2
• Higher urinary concentrations (500-1000 μg/ml) are achieved following 250-500 mg oral doses, which are many times greater than the MIC for common urinary pathogens 3

Respiratory Tract Infections

• Adults: 500 mg four times daily 1
• Children: 25-50 mg/kg/day in 4 divided doses 1

Clinical Considerations

Duration of Therapy

• 7-10 days for most infections, depending on clinical response 1
• 5-7 days for uncomplicated urinary tract infections 2

Pharmacokinetic Considerations

• Cephalexin is not absorbed from the stomach but is totally and rapidly absorbed in the upper intestine 3
• No measurable destruction or metabolism occurs during its time in the body fluids 3
• 70-100% of the dose is found in the urine 6-8 hours after each dose 3
• Children may require higher doses per kilogram than adults due to greater body water turnover 3

Common Side Effects

• Diarrhea (most common) 6
• Nausea 6
• Vaginal mycosis in females 6

Important Caveats

• Cephalexin does not have activity against MRSA despite some studies showing clinical response, which may be due to other interventions like drainage 6
• Dose adjustments are necessary in patients with significant renal impairment 3, 4
• Twice-daily dosing regimens may improve treatment adherence compared to four-times-daily dosing 5, 2