What is the recommended dose of Keflex (cephalexin) for treating urinary tract infections (UTIs)?

Cephalexin Dosing for Urinary Tract Infections

For uncomplicated urinary tract infections (UTIs), cephalexin should be dosed at 500 mg four times daily for 5-7 days, though 500 mg twice daily may be an equally effective alternative regimen with potentially better adherence. 1, 2, 3, 4

Recommended Dosing Regimens

Adult Dosing

• Standard dosing: 500 mg four times daily for 5-7 days 2
• Alternative dosing: 500 mg twice daily for 5-7 days 3, 4
  • Recent evidence suggests twice-daily dosing is as effective as four-times-daily dosing for uncomplicated UTIs 3, 4
  • Twice-daily dosing may improve patient adherence

Pediatric Dosing

• 25-50 mg/kg/day divided into four doses 2
• For children weighing 40 kg (88 lb): 2-4 teaspoons (250 mg/5 mL) four times daily 2

Treatment Duration

• Uncomplicated UTIs: 5-7 days 1, 3
• Complicated UTIs: 7-14 days 2

Clinical Considerations

Advantages of Cephalexin for UTIs

• Excellent urinary penetration with high concentrations (500-1000 μg/mL) in urine following standard doses 5
• Effective against common UTI pathogens including E. coli, which accounts for approximately 85% of UTIs 3
• Low incidence of side effects 5
• Complete absorption in the upper intestine 5

Special Populations

• Renal impairment: Dose reduction required for patients with creatinine clearance <30 mL/min 5
• Pregnancy: Cephalexin is considered a safe option during pregnancy 1

Treatment Failure Considerations

• If symptoms worsen or do not improve within 72 hours, reassessment is necessary 1
• Obtain urine cultures before initiating antibiotics to guide therapy if initial treatment fails 1

Comparative Effectiveness

• Recent studies show no significant difference in treatment failure rates between twice-daily (18.7%) and four-times-daily (15.0%) dosing regimens (p=0.465) 4
• For uncomplicated UTIs specifically, treatment failure rates were 14.9% for twice-daily vs 8.1% for four-times-daily dosing (p=0.197) 4
• No significant differences in adverse events between dosing regimens 3

Practical Recommendations

1. For most adult patients with uncomplicated UTIs, start with cephalexin 500 mg twice daily for 5-7 days
2. For complicated UTIs or severe infections, use 500 mg four times daily for 7-14 days
3. Complete the full course of antibiotics even if symptoms improve quickly
4. Do not perform surveillance urine cultures in asymptomatic patients after treatment

While cephalexin is an effective option for UTIs, it's worth noting that nitrofurantoin (100 mg twice daily for 5 days) and trimethoprim-sulfamethoxazole (160/800 mg twice daily for 3 days) are considered first-line agents for uncomplicated UTIs according to current guidelines, with cephalexin being an alternative option 1.