What is the typical dosage of Cephalexin for treating bacterial infections?

Cephalexin Dosing for Bacterial Infections

For adults with skin and soft tissue infections, cephalexin 500 mg four times daily is the standard dose, while urinary tract infections typically require 250 mg every 6 hours or 500 mg every 12 hours, and pediatric dosing ranges from 25-50 mg/kg/day divided into doses. 1

Adult Dosing

Skin and Soft Tissue Infections

• Standard dose: 500 mg four times daily (QID) orally for methicillin-susceptible staphylococcal and streptococcal infections 2
• Alternative dosing: 500 mg every 6 hours for routine infections 1
• The total daily adult dosage ranges from 1-4 grams divided throughout the day 1
• Treatment duration is typically 7 days, adjusted based on clinical response 2, 3

Urinary Tract Infections

• Uncomplicated cystitis: 500 mg every 12 hours for patients over 15 years of age 1
• Alternative: 250 mg every 6 hours for less severe infections 1
• Continue therapy for 7-14 days 1
• Single-dose therapy with 3 grams has shown 67% cure rates in acute uncomplicated UTI, though this is not standard practice 4

Streptococcal Pharyngitis

• 500 mg every 12 hours is adequate for streptococcal throat infections 1
• Must continue for at least 10 days to eradicate β-hemolytic streptococci 1

Pediatric Dosing

Standard Dosing

• Usual dose: 25-50 mg/kg/day divided into doses 1
• For streptococcal pharyngitis and skin infections in children >1 year: may divide total daily dose every 12 hours 1
• For severe infections: double the standard dosage 1

Specific Pediatric Indications

• Otitis media requires 75-100 mg/kg/day in 4 divided doses for adequate treatment 1
• For febrile infants 29-60 days old with UTI: 50-100 mg/kg/day in 4 doses 2
• β-hemolytic streptococcal infections require at least 10 days of therapeutic dosing 1

Weight-Based Dosing Examples (using suspension)

For 250 mg/5 mL suspension given four times daily 1:

• 10 kg child: 1/4 to 1/2 teaspoon QID
• 20 kg child: 1/2 to 1 teaspoon QID
• 40 kg child: 1 to 2 teaspoons QID

Important Clinical Considerations

Spectrum of Activity

• Cephalexin is effective against methicillin-susceptible Staphylococcus aureus (MSSA) and Streptococcus species 2, 3
• NOT effective against MRSA - alternative agents must be used if MRSA is suspected or confirmed 3, 5
• Limited activity against Pasteurella multocida and anaerobes 3
• Poor activity against Haemophilus influenzae - expect 50% failure rate in H. influenzae infections 6

Dosing Adjustments

• Patients with creatinine clearance <30 mL/min require dose reduction proportional to renal function 7
• More severe infections or less susceptible organisms may require larger doses 1
• If >4 grams daily is needed, consider switching to parenteral cephalosporins 1

Common Pitfalls to Avoid

• Do not use cephalexin for suspected MRSA infections - this is a critical error that leads to treatment failure 3, 5
• Do not use for H. influenzae otitis media or respiratory infections - high failure rates 6
• Patients with immediate penicillin hypersensitivity should avoid cephalexin due to cross-reactivity risk 3
• Inadequate duration for streptococcal infections (<10 days) risks treatment failure and rheumatic fever 1

Monitoring

• If no improvement within 5 days, extend treatment or broaden coverage 3
• Watch for systemic signs requiring broader antimicrobial coverage 3
• Adverse effects (mostly GI upset, rash) are infrequent and rarely require discontinuation 6

Pharmacokinetic Advantages

• Rapid and complete absorption from upper intestine 7
• Achieves urinary concentrations of 500-1000 mcg/mL after 250-500 mg doses - far exceeding MIC for typical uropathogens 7
• 70-100% excreted unchanged in urine within 6-8 hours 7
• Does not disturb lower bowel flora due to high intestinal absorption 7