What are the uses and dosing guidelines for cephalexin in treating bacterial infections?

Cephalexin Uses and Dosing Guidelines

Cephalexin is a first-generation oral cephalosporin antibiotic primarily used to treat skin and soft tissue infections, respiratory tract infections, urinary tract infections, and bone infections caused by methicillin-susceptible Staphylococcus aureus (MSSA) and Streptococcus species. 1

Primary Clinical Indications

Skin and Soft Tissue Infections

• The standard adult dose is 500 mg orally every 6 hours (four times daily) for MSSA skin infections, as recommended by the Infectious Diseases Society of America 2, 3
• For surgical site infections following clean procedures (trunk or extremity surgery away from axilla/perineum), cephalexin 500 mg every 6 hours orally is an appropriate option 2
• Cephalexin achieves cure rates of 90% or higher for streptococcal and staphylococcal skin infections 4
• Treatment duration is typically 7-10 days depending on clinical response 3, 5

Respiratory Tract Infections

• For streptococcal pharyngitis and tonsillitis, cephalexin is comparable to penicillin in efficacy 6
• Adult dosing: 500 mg every 12 hours for streptococcal pharyngitis 1
• Critical limitation: Cephalexin has poor activity against Haemophilus influenzae, making it inappropriate for sinusitis, many community-acquired pneumonias, and otitis media where H. influenzae is suspected 7, 6, 3
• For beta-hemolytic streptococcal infections, treatment must continue for at least 10 days to prevent rheumatic fever 1, 3

Urinary Tract Infections

• For uncomplicated cystitis in patients over 15 years: 500 mg every 12 hours for 7-14 days 1
• Cephalexin achieves urinary concentrations of 500-1000 mcg/mL following 250-500 mg oral doses, far exceeding minimum inhibitory concentrations for common uropathogens 8
• Single-dose therapy (3 g) shows 67% cure rates for acute uncomplicated UTIs, though this is not standard practice 9

Dosing by Patient Population

Adult Dosing

• Standard range: 1-4 grams daily in divided doses 1
• Usual dose: 250 mg every 6 hours for mild infections 1
• For moderate-to-severe infections: 500 mg every 6 hours 1, 3
• If doses exceeding 4 grams daily are required, switch to parenteral cephalosporins (e.g., cefazolin 1 g IV every 8 hours) 10

Pediatric Dosing

• Standard: 25-50 mg/kg/day divided into 4 doses for mild-to-moderate infections 1, 3
• For MSSA infections: 75-100 mg/kg/day divided into 3-4 doses 3, 10
• For otitis media: 75-100 mg/kg/day in 4 divided doses is required 1
• For streptococcal pharyngitis and skin infections in children over 1 year, the total daily dose may be divided every 12 hours 1

Renal Impairment

• Patients with creatinine clearance <30 mL/min require dose reduction proportional to reduced renal function 8

Antimicrobial Spectrum and Critical Limitations

Effective Coverage

• Methicillin-susceptible Staphylococcus aureus (MSSA) 3, 10, 4
• Streptococcus pyogenes (Group A Streptococcus) 6, 4
• Other beta-hemolytic streptococci 1, 6

Critical Coverage Gaps (Common Pitfalls)

• Completely ineffective against MRSA - if MRSA is suspected, use trimethoprim-sulfamethoxazole, doxycycline, or clindamycin instead 10, 5, 3
• Poor activity against Haemophilus influenzae - do not use for sinusitis, otitis media, or respiratory infections where H. influenzae is likely 7, 6, 3
• Inadequate against Moraxella catarrhalis (beta-lactamase producing strains) 3
• No activity against Pseudomonas aeruginosa 3
• Limited anaerobic coverage - unsuitable for deep abscesses or infections requiring anaerobic coverage 10
• Poor coverage for Pasteurella multocida (animal bites) and Eikenella corrodens (human bites) 10
• High resistance rates (96%) among viridans group streptococci - not recommended for endocarditis prophylaxis 3

Special Clinical Considerations

Penicillin Allergy

• Cephalexin can be used in patients with non-severe penicillin allergy (non-IgE mediated reactions) 10, 5
• Contraindicated in patients with history of anaphylaxis, angioedema, or urticaria to penicillins due to potential cross-reactivity 3, 5

Pregnancy and Lactation

• For lactating women with mastitis: 500 mg orally every 6 hours 3
• Cephalexin achieves therapeutic levels in breast milk (0.09-0.59 mcg/mL in healthy milk; 0.57-1.05 mcg/mL in infected milk) and is considered safe for nursing infants 3

Administration and Storage

• Cephalexin is absorbed in the upper intestine, not the stomach, ensuring reliable bioavailability 8
• Does not disturb lower bowel flora due to absorption high in the intestinal tract 8
• Suspension must be refrigerated after mixing and remains stable for 14 days 1

Algorithm for Appropriate Use

Step 1: Identify the infection type and likely pathogens

• Skin/soft tissue with suspected MSSA/Streptococcus → Cephalexin appropriate
• Respiratory infection with possible H. influenzae → Choose alternative agent
• Suspected MRSA → Choose alternative agent (TMP-SMX, doxycycline, clindamycin)

Step 2: Assess patient factors

• Penicillin allergy history: severe (anaphylaxis/angioedema) → avoid cephalexin; non-severe → cephalexin acceptable
• Renal function: CrCl <30 mL/min → reduce dose proportionally

Step 3: Select appropriate dose

• Adults: 500 mg every 6 hours for most infections
• Children: 25-50 mg/kg/day (mild) or 75-100 mg/kg/day (MSSA) in divided doses
• Duration: 7-10 days (minimum 10 days for streptococcal infections)

Step 4: Monitor for treatment failure

• If no improvement by day 5, consider MRSA, resistant organisms, or alternative diagnosis 5