What is the recommended dosage and coverage of cephalexin for a typical adult patient with no significant medical history?

Cephalexin Dosing and Antimicrobial Coverage

For a typical adult with no significant medical history, cephalexin is dosed at 500 mg orally every 6 hours (four times daily) for most infections, providing excellent coverage against methicillin-susceptible Staphylococcus aureus (MSSA) and beta-hemolytic streptococci, but it is completely ineffective against MRSA, has poor activity against Haemophilus influenzae and Moraxella catarrhalis, and provides no coverage for Pseudomonas aeruginosa. 1, 2, 3

Standard Adult Dosing Regimens

For skin and soft tissue infections:

• 500 mg orally every 6 hours (four times daily) for 5 days if clinical improvement occurs 1, 2, 3
• Extend treatment beyond 5 days only if symptoms have not improved within this timeframe 1, 4
• This dosing provides adequate tissue concentrations against MSSA and streptococcal species 1, 2

For uncomplicated urinary tract infections:

• 500 mg every 12 hours (twice daily) is as effective as four-times-daily dosing and may improve adherence 3, 5
• Treatment duration should be 7-14 days for cystitis 3

For streptococcal pharyngitis:

• 500 mg every 12 hours is acceptable 3
• Must be administered for at least 10 days to prevent rheumatic fever 1, 3

For severe infections requiring higher doses:

• If daily doses exceeding 4 grams are required, switch to parenteral cephalosporins (e.g., cefazolin 1-2 g IV every 8 hours) 1, 3

Antimicrobial Spectrum and Critical Coverage Gaps

Gram-positive coverage (excellent):

• Methicillin-susceptible Staphylococcus aureus (MSSA) 1, 2, 6
• Beta-hemolytic streptococci, especially Streptococcus pyogenes 1, 2, 6
• Most streptococcal species except those with high resistance rates 1

Critical coverage gaps (avoid cephalexin for these pathogens):

• Completely ineffective against MRSA - if MRSA is suspected, use trimethoprim-sulfamethoxazole, doxycycline, or clindamycin instead 1, 2
• Poor activity against Haemophilus influenzae - makes it inappropriate for sinusitis, otitis media (50% failure rate), and many respiratory infections 1, 7, 8
• Inadequate activity against beta-lactamase-producing Moraxella catarrhalis 1
• No activity against Pseudomonas aeruginosa 1
• Limited coverage against anaerobes - unsuitable for deep abscesses or anaerobic infections 1
• Poor activity against Pasteurella multocida (animal bites) and Eikenella corrodens (human bites) 1

When Cephalexin Is Appropriate vs. When It's Not

Use cephalexin for:

• Uncomplicated nonpurulent cellulitis without MRSA risk factors 2, 4
• Skin and soft tissue infections caused by MSSA or streptococci 1, 2
• Uncomplicated urinary tract infections with susceptible organisms 3, 5, 9
• Streptococcal pharyngitis (alternative to penicillin) 3, 7

Do NOT use cephalexin for:

• Cellulitis with purulent drainage, penetrating trauma, or injection drug use (MRSA risk factors present) 2, 4
• Acute bacterial sinusitis (poor H. influenzae coverage) 1, 7
• Otitis media in children (50% failure rate with H. influenzae) 1, 7
• Animal or human bite wounds (requires broader coverage) 1
• Infective endocarditis prophylaxis (96% resistance among viridans group streptococci) 1
• Any infection where MRSA is suspected or confirmed 1, 2

Pediatric Dosing

Standard dosing:

• 25-50 mg/kg/day divided into 4 doses for mild to moderate infections 1, 3
• 75-100 mg/kg/day divided into 3-4 doses for MSSA infections 1, 2, 3
• For otitis media, 75-100 mg/kg/day in 4 divided doses is required 3

Streptococcal pharyngitis and skin infections:

• May divide total daily dose and administer every 12 hours in patients over 1 year of age 3
• Must treat for at least 10 days for streptococcal infections 1, 3

Pharmacokinetic Considerations

• Cephalexin is totally and rapidly absorbed in the upper intestine (not from the stomach) 6
• Achieves urinary concentrations of 500-1000 mcg/mL following 250-500 mg oral doses 6, 9
• 70-100% of the dose is excreted unchanged in urine within 6-8 hours 6, 9
• Low serum protein binding and no measurable metabolism 6
• Does not penetrate into host tissue cells, accounting for low incidence of side effects 6
• Patients with creatinine clearance <30 mL/min require dose reduction proportional to reduced renal function 6

Common Pitfalls to Avoid

1. Do not use cephalexin for typical cellulitis if MRSA risk factors are present (purulent drainage, penetrating trauma, injection drug use, known MRSA colonization) 2, 4
2. Do not extend treatment to 7-10 days based on tradition alone - 5 days is sufficient for uncomplicated infections if clinical improvement occurs 1, 4
3. Do not use for respiratory infections where H. influenzae is likely - resistance reduces effectiveness in pediatric populations and some adults with chronic bronchitis 1, 7, 8
4. Do not use for meningitis - cephalexin produces little or no cerebrospinal fluid levels 8
5. Avoid in patients with confirmed immediate-type amoxicillin allergy - cephalexin shares identical R1 side chains with amoxicillin 2