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Sunday, July 27, 2025

Cefalexin


Cefalexin (also spelled cephalexin) is a first-generation cephalosporin antibiotic used primarily to treat a range of mild to moderate bacterial infections. It is structurally and pharmacologically related to penicillins and functions by interfering with bacterial cell wall synthesis, making it particularly effective against gram-positive organisms and some gram-negative bacteria.

This detailed professional monograph covers cefalexin’s classification, mechanism of action, spectrum, clinical indications, dosing protocols, contraindications, adverse effects, precautions, and drug interactions.

Pharmacological Classification

  • Therapeutic class: Antibacterial

  • Pharmacologic class: First-generation cephalosporin (β-lactam antibiotic)

  • ATC Code: J01DB01

Brand Names and Formulations

  • Keflex®, Ceporex®, Rilexine®, Sporidex®, Biocef®

  • Available generically as cefalexin or cephalexin

  • Oral formulations:

    • Capsules: 250 mg, 500 mg

    • Tablets: 250 mg, 500 mg

    • Oral suspension: 125 mg/5 mL, 250 mg/5 mL

  • Not available for intravenous or intramuscular administration

Mechanism of Action

Cefalexin binds to penicillin-binding proteins (PBPs) inside the bacterial cell wall, inhibiting the final transpeptidation step in the synthesis of the peptidoglycan layer. This weakens the cell wall, resulting in cell lysis and bactericidal activity.

Its action is time-dependent and most effective when serum levels remain above the minimum inhibitory concentration (MIC) for the infecting organism.

Antimicrobial Spectrum

Effective Against

Gram-Positive Bacteria

  • Streptococcus pyogenes (Group A β-hemolytic streptococci)

  • Streptococcus pneumoniae (limited susceptibility)

  • Staphylococcus aureus (MSSA only)

  • Staphylococcus epidermidis (methicillin-sensitive)

Gram-Negative Bacteria

  • Escherichia coli

  • Klebsiella pneumoniae

  • Proteus mirabilis

  • Shigella spp.

  • Salmonella spp.

Not Effective Against

  • MRSA (methicillin-resistant S. aureus)

  • Pseudomonas aeruginosa

  • Enterococcus spp.

  • Bacteroides fragilis

  • Acinetobacter spp.

  • Extended-spectrum beta-lactamase (ESBL) producing organisms

Clinical Indications

Licensed Uses

  • Upper respiratory tract infections (URTIs): pharyngitis, tonsillitis

  • Lower respiratory tract infections (LRTIs): bronchitis, mild pneumonia

  • Otitis media

  • Urinary tract infections (UTIs): cystitis, urethritis

  • Skin and soft tissue infections (SSTIs): impetigo, cellulitis, wound infections

  • Bone infections: osteomyelitis

  • Dental infections: abscesses, prophylaxis in penicillin-allergic patients (non-anaphylactic)

Off-label Uses

  • Endocarditis prophylaxis in dental procedures (when penicillin allergy is absent)

  • Uncomplicated prostatitis (alternative therapy)

  • Uncomplicated gonorrhea in combination therapies (rare)

Dosage and Administration

Adults and Children ≥12 Years

  • 250 mg to 1 g every 6 hours, or 500 mg every 12 hours

  • Usual daily dose: 1–4 g depending on infection severity

  • Maximum: 4 g/day

Children (1 month–12 years)

  • 25–50 mg/kg/day in divided doses every 6–12 hours

  • In severe infections: up to 100 mg/kg/day

Infants <1 month

  • Use with caution; specialist dosing required

Renal Impairment

  • Dose adjustment required:

    • CrCl 10–50 mL/min: administer every 8–12 hours

    • CrCl <10 mL/min: administer every 12–24 hours

  • Hemodialysis: dose post-dialysis

Treatment Duration

  • Uncomplicated UTI: 5–7 days

  • Pharyngitis/tonsillitis: 10 days

  • SSTIs: 7–10 days

  • Osteomyelitis: up to 6 weeks (oral step-down)

Pharmacokinetics

  • Absorption: ~90% orally bioavailable

  • Peak plasma concentration: 1 hour post-dose

  • Half-life: ~1 hour

  • Metabolism: Not metabolized extensively

  • Excretion: Primarily renal (~80–100% unchanged in urine)

  • Crosses placenta, appears in breast milk

Contraindications

  • Hypersensitivity to cefalexin, cephalosporins, or any excipient

  • History of immediate or severe allergic reactions (e.g., anaphylaxis) to penicillins (relative contraindication due to cross-reactivity)

  • Caution in patients with porphyria

Warnings and Precautions

  • Penicillin allergy: 5–10% cross-reactivity with cephalosporins

  • Renal impairment: Accumulation risk; adjust dose

  • Clostridioides difficile–associated diarrhea (CDAD): Risk with any antibiotic, including cefalexin

  • Prolonged use: Can result in fungal or bacterial superinfection, e.g., candidiasis, resistant strains

  • Coombs’ test may become falsely positive during therapy

  • Seizure risk in high doses or renal impairment

Adverse Effects

Common (≥1%)

  • Gastrointestinal: diarrhea, nausea, vomiting, abdominal pain

  • Skin reactions: rash, urticaria

  • Vaginal candidiasis

Uncommon (<1%)

  • Dizziness, headache, fatigue

  • Eosinophilia, transient hepatic enzyme elevation

  • Pruritus, photosensitivity

Rare (<0.1%)

  • Anaphylaxis, Stevens-Johnson syndrome, toxic epidermal necrolysis

  • Neutropenia, thrombocytopenia, hemolytic anemia

  • Hepatitis, cholestatic jaundice

  • Seizures (usually in overdose or renal failure)

  • Pseudomembranous colitis (C. difficile)

Drug Interactions

  • Probenecid: Decreases renal excretion → ↑ cefalexin levels

  • Metformin: Risk of lactic acidosis if renal function is impaired

  • Oral contraceptives: No consistent evidence of reduced efficacy, but some clinicians recommend backup contraception

  • Loop diuretics and aminoglycosides: Increased nephrotoxicity risk

  • Anticoagulants: Monitor INR closely in warfarin-treated patients; some cephalosporins enhance bleeding risk

Pregnancy and Lactation

Pregnancy (Category B)

  • No teratogenic effects in animal studies

  • Generally considered safe for use in all trimesters

  • Often used in UTIs during pregnancy

Lactation

  • Excreted in breast milk in small quantities

  • No known adverse effects in nursing infants

  • Monitor for GI upset or thrush

Use in Special Populations

  • Pediatrics: Widely used and well-tolerated; adjust dose based on weight

  • Elderly: Monitor renal function; no special dosage unless renal impairment

  • Renal impairment: Dose adjustment crucial

  • Hepatic impairment: No adjustment required

Counseling Points

  • Take with or without food; food may reduce GI side effects

  • Complete the full course, even if symptoms improve

  • Shake oral suspensions well before use; store refrigerated (if applicable)

  • Discard suspension after 14 days

  • Report severe diarrhea, rash, or allergic symptoms

  • Maintain adequate hydration

  • Keep medication out of reach of children

  • Inform healthcare provider about penicillin or cephalosporin allergies

Clinical Considerations

  • Excellent choice for skin infections caused by Streptococcus and MSSA

  • In uncomplicated cystitis, may be used as a second-line agent if nitrofurantoin or trimethoprim is unsuitable

  • Not effective for beta-lactamase–producing organisms

  • Avoid empirical use in regions with high resistance rates in E. coli

  • Not suitable for anaerobic infections or Pseudomonas coverage

  • Used in step-down therapy after IV cefazolin or ceftriaxone




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