Cephalexin (Keflex) versus Amoxicillin

Based on "Antibiotic and Chemotherapy"
written by Roger G. Finch

Difference between Cephalexin and Amoxicillin

  • Cephalexin is a first-line choice for uncomplicated cellulitis and impetigo because it has very good activity against streptococci and methicillin-sensitive S. aureus 4.
  • Cephalexin is a suitable choice for the treatment of streptococcal pharyngitis in patients with delayed-reaction penicillin allergy.
  • Amoxicillin is a first-line antibiotic for acute otitis media, sinusitis, and streptococcal pharyngitis.
  • Amoxicillin is a preferred medication for endocarditis prophylaxis.
Cephalexin Amoxicillin
Keflex® Amoxil®
Drug class
Cephalosporin antibiotic Penicillin antibiotic
Dose formulations
• Capsules
• Oral suspension
• Tablets
• Capsules
• Oral suspension
• Tablets
• Tablets, chewable
• Tablets, extended release
FDA-approved indications
• Otitis media
• Streptococcal pharyngitis
• Skin and skin structure infections
• Sinusitis
• Bone infections
• Cystitis
• Acute prostatitis
• Uncomplicated pneumonia
• Sinusitis
• In combination for treatment of H. pylori infection and duodenal ulcer disease.
"Off-label" uses
• Infective endocarditis (prophylaxis)
• Appendicular peritonitis • Anthrax, inhalational post-exposure prophylaxis
• Erysipeloid
• Lyme disease
Chlamydia in pregnant women
Mechanism of action
• Bactericidal
• Inhibits the synthesis of bacterial cell walls by binding to penicillin-binding proteins.
• 0.5 to 1.2 hours • Immediate-release: ~ 60 minutes
• Extended-release: 90 minutes
Oral bioavailability
• 90% • 74-92%
Metabolism, Elimination
• Cephalexin is minimally metabolized.
• Cephalexin is eliminated in urine (80% to 100% as unchanged drug).
• Amoxicillin is eliminated
in urine (60% as unchanged drug).
• Hypersensitivity to cephalexin
• Hypersensitivity to other cephalosporins
• Hypersensitivity to amoxicillin
• Hypersensitivity to penicillins or cephalosporins
Side effects
• Diarrhea
• Nausea
• Vomiting
• Upset stomach
• Abdominal pain

• Cephalexin may increase metformin levels
• Diarrhea
• Nausea
• Vomiting
• Rash
• Abdominal pain
• Serious allergic reactions (anaphylaxis, serum sickness-like reaction)

Cephalexin vs Amoxicillin for tooth abscess

Dental abscess (also termed abscessed tooth) represent localized collection of pus at the tooth root apex. Dental abscesses are best managed by operative intervention such as incisional drainage, root canal, or extraction.

Antibiotics are used as supportive measure for controlling the spread of infection and preventing further complications.


Amoxicillin is considered a first-line choice for dental infections1. Amoxicillin is bactericidal, has a fairly narrow antimicrobial spectrum, but cover most bacteria responsible for dental infections.

The addition of clavulanic acid (a beta-lactamase inhibitor) makes amoxicillin active against most beta-lactamase-producing bacteria. Amoxicillin/clavulanic acid can be used to treat severe dental infection with spreading cellulitis or dental infection that has not responded to first-line treatment.

Cephalexin is useful in the treatment of dental infections 2. The efficacy rate is about 92%. Cephalexin perfectly penetrates alveolar bone3.

However, unlike amoxicillin it is NOT a first-line treatment for abscessed tooth. Cephalexin is most effective against streptococci and staphylococci, but this antibiotic does not cover aerobic gram-negative bacteria.

Further reading


  • 1. Gonzalez-Martinez R, Cortell-Ballester I, Herraez-Vilas JM, Arnau-de Bolos JM, Gay-Escoda C. Antibiotic prescription in the treatment of odontogenic infection by health professionals: a factor to consensus. Med Oral Patol Oral Cir Bucal. 2012 May 1;17(3):e452-6.
  • 2. Horii M, Morinaga T, Takeuchi T, et al. A double-blind comparison between cefaclor and cephalexin in the treatment of dental infections. Jpn J Antibiot. 1984 Jan;37(1):152-75.
  • 3. Shuford GM. Concentrations of cephalexin in mandibular alveolar bone, blood, and oral fluids. J Am Dent Assoc. 1979 Jul;99(1):47-50.
  • 4. IDSA Practice Guidelines for the Diagnosis and Management of Skin and Soft Tissue Infections. Full text

Published: March 31, 2008
Last reviewed: November 05, 2017


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