Cephalexin 500 mg: Indications, Dosage, Contraindications, Off-Label Uses, and Side Effects
Cephalexin 500 mg is a first-generation cephalosporin indicated for respiratory tract infections, otitis media, skin and soft tissue infections, bone infections, and genitourinary tract infections caused by susceptible organisms, with standard adult dosing of 500 mg every 6 hours orally. 1
FDA-Approved Indications
Cephalexin is indicated for infections caused by susceptible strains of specific microorganisms 1:
- Respiratory tract infections: Caused by Streptococcus pneumoniae and Streptococcus pyogenes
- Otitis media: Due to S. pneumoniae, Haemophilus influenzae, Staphylococcus aureus, S. pyogenes, and Moraxella catarrhalis
- Skin and soft tissue infections: Caused by S. aureus and/or S. pyogenes
- Bone infections: Caused by S. aureus and/or Proteus mirabilis
- Genitourinary tract infections: Including acute prostatitis, caused by Escherichia coli, P. mirabilis, and Klebsiella pneumoniae
Important caveat: While penicillin remains the drug of choice for streptococcal infections including rheumatic fever prophylaxis, cephalexin is generally effective for streptococcal eradication from the nasopharynx, though data for rheumatic fever prevention are limited 1.
Dosage
Adults
- Standard dose: 500 mg every 6 hours orally 2
- Alternative: 250-500 mg every 6 hours orally for less severe infections 2
- High-dose regimen: 1000 mg four times daily has been studied for cellulitis with potentially lower treatment failure rates (3.2% vs 12.9%), though with more minor adverse effects 3
- Duration: Typically 7 days, though 5 days may be sufficient for cellulitis if clinical improvement occurs 2
Pediatric Dosing
- Standard: 25-50 mg/kg/day in divided doses 4
- High-dose for bone/joint infections: 45 mg/kg/dose (maximum 1.5 g) three times daily has demonstrated 99% cure rates 5
- Alternative: 25 mg/kg/dose (maximum 750 mg/dose) three times daily achieves pharmacodynamic targets for MSSA 6
Renal Impairment
Dose reduction is required when creatinine clearance is less than 30 mL/min, proportional to reduced renal function 4. Cephalexin is 70-100% renally excreted unchanged within 6-8 hours 4.
Contraindications
Cephalexin is absolutely contraindicated in patients with known allergy to the cephalosporin group of antibiotics 1.
Cross-Reactivity with Penicillin Allergy
- Less than 5% of patients with penicillin allergy labels have true penicillin allergy 7
- Cephalexin can be used as first-line prophylaxis even in patients with documented penicillin allergy, including anaphylaxis 7
- Exception: Higher cross-reactivity risk exists with amino-penicillin allergies (amoxicillin, ampicillin) due to shared R1 side chains 7
- Historical data show 462 penicillin-sensitive patients were treated with cephalexin with only 21 developing sensitivity reactions (4.5%) 8
Off-Label Uses
While not explicitly FDA-approved, cephalexin is used off-label for:
- Recurrent S. aureus abscesses: Part of a decolonization regimen including intranasal mupirocin and chlorhexidine washes 2
- Surgical prophylaxis: Used in dermatologic surgery, including in penicillin-allergic patients 7
- Cellulitis treatment: Recommended by IDSA guidelines for non-purulent cellulitis as first-line therapy targeting streptococci 2
Clinical context from guidelines: For mild cellulitis without systemic signs, cephalexin provides adequate streptococcal coverage. For moderate infections with systemic signs, many clinicians include MSSA coverage (which cephalexin provides). For severe infections with MRSA risk factors, alternative agents are needed 2.
Side Effects
Common Adverse Events (6% overall incidence, 3% definite/probable drug-related) 8
Gastrointestinal (most frequent) 1, 8:
- Diarrhea (most common, reported by 164/12,917 patients in clinical trials)
- Nausea and vomiting (rare)
- Dyspepsia, gastritis, abdominal pain
- Pseudomembranous colitis (can occur during or after treatment)
Hypersensitivity Reactions 1:
- Rash, urticaria, angioedema
- Erythema multiforme, Stevens-Johnson syndrome, toxic epidermal necrolysis (rare)
- Anaphylaxis (reported)
- Genital and anal pruritus, genital moniliasis, vaginitis
Hematologic 1:
- Eosinophilia
- Neutropenia (13/149 children in one study, mostly mild) 5
- Thrombocytopenia
- Hemolytic anemia (rare but can be profound, requiring transfusions) 9
Hepatic 1:
- Transient hepatitis and cholestatic jaundice (rare)
- Slight elevations in AST and ALT
Neurologic 1:
- Dizziness, fatigue, headache
- Agitation, confusion, hallucinations
- Seizures (particularly in renal impairment with inadequate dose reduction)
Renal 1:
- Reversible interstitial nephritis (rare)
Other 1:
- Arthralgia, arthritis, joint disorder
Timing and Risk Factors
- Most adverse events occur 6-15 days after treatment initiation 8
- Patients at extremes of age (very young and elderly) report more events 8
- Four-times-daily dosing associated with more events than twice-daily dosing 8
- High-dose regimens (1000 mg four times daily) have higher rates of minor adverse effects 3
Critical Safety Consideration
Cephalexin can cause profound drug-induced immune hemolytic anemia (DIIHA) requiring transfusions and prolonged hospitalization, even at typical doses 9. While rare, this represents a serious adverse event that requires early recognition and immediate drug discontinuation.