What is the dosage and treatment regimen for cephalexin 500 mg in an adult with a bacterial infection?

Cephalexin 500 mg Dosing and Treatment Regimen

For most bacterial infections in adults, cephalexin 500 mg should be administered four times daily (every 6 hours) for 7-10 days, with the specific duration and frequency adjusted based on infection type and severity. 1, 2

Standard Adult Dosing Regimens

Most Common Infections

• 500 mg every 6 hours (four times daily) is the standard dose for skin and soft tissue infections, particularly those caused by methicillin-susceptible Staphylococcus aureus (MSSA) 3, 1, 4
• The FDA-approved adult dosage range is 1-4 grams daily in divided doses 2
• For severe infections requiring more than 4 grams daily, switch to parenteral cephalosporins (e.g., cefazolin 1 gram IV every 8 hours) 1, 2

Alternative Dosing for Specific Infections

• 500 mg every 12 hours (twice daily) may be used for:
  • Streptococcal pharyngitis 2
  • Uncomplicated skin and skin structure infections 2
  • Uncomplicated cystitis in patients over 15 years (treat for 7-14 days) 2
  • Recent evidence supports twice-daily dosing for uncomplicated urinary tract infections with similar efficacy to four-times-daily dosing 5

Infection-Specific Considerations

Skin and Soft Tissue Infections:

• 500 mg four times daily for 7-10 days 1, 4, 6
• Provides excellent coverage against MSSA and Streptococcus species 4

Streptococcal Infections:

• Must be administered for at least 10 days to prevent rheumatic fever 1, 2

Respiratory Tract Infections:

• 500 mg four times daily 1
• Critical pitfall: Cephalexin has poor activity against Haemophilus influenzae and Moraxella catarrhalis, making it inappropriate for sinusitis and many respiratory infections 1

Timing and Administration

• Administer every 6 hours (e.g., 6:00 AM, 12:00 PM, 6:00 PM, 12:00 AM) to maintain consistent antibiotic levels 1
• Cephalexin is absorbed high in the intestinal tract and can be taken without regard to meals 7
• Absorption is total and rapid in the upper intestine, with 70-100% of the dose appearing in urine within 6-8 hours 7

Critical Limitations and When NOT to Use Cephalexin

Completely Ineffective Against:

• MRSA (methicillin-resistant Staphylococcus aureus) - use trimethoprim-sulfamethoxazole, doxycycline, or clindamycin instead 1, 4, 6
• Pseudomonas aeruginosa 1
• Most anaerobes (unsuitable for deep abscesses or anaerobic infections) 1, 4

Poor Activity Against:

• Haemophilus influenzae 1
• Moraxella catarrhalis (beta-lactamase-producing strains) 1
• Pasteurella multocida (animal bites) 1, 4
• Eikenella corrodens (human bites) 1

Not Recommended For:

• Endocarditis prophylaxis (96% resistance among viridans group streptococci) 1
• Acute bacterial sinusitis 1
• Bite wounds as monotherapy 1

Allergy Considerations

• Patients with immediate hypersensitivity reactions (anaphylaxis, angioedema, urticaria) to penicillins should avoid cephalexin due to cross-reactivity 4, 6
• For non-immediate penicillin reactions, cephalexin remains an appropriate alternative 4
• Cephalexin causes a very low incidence of allergy due to its stability and chemical configuration 7

Renal Dosing Adjustments

• Patients with creatinine clearance less than 30 mL/min require dosage reduction proportional to reduced renal function 7
• Cephalexin is rapidly cleared by the kidneys, with urinary concentrations of 500-1000 mcg/mL following 250-500 mg oral doses 7

Monitoring and Follow-up

• If infection has not improved within 5 days, extend treatment duration 6
• If systemic signs of infection develop or worsen, consider broadening antimicrobial coverage 6
• Typical treatment duration is 7-10 days for most infections, depending on clinical response 1, 6