Cephalexin (Keflex®)

Cephalexin in Brief
  • Generic name: cephalexin
  • Brand names: Keflex,® Keftabs, Biocef
  • Therapeutic class: Antibiotic
  • Pharmacologic class: Cephalosporin, first-generation
  • Pregnancy Category: B
  • FDA Approved: January 4, 1971
  • Chemical Formula: C16H17N3O4S
  • Originally discovered: 1967, Eli Lilly Ltd, USA

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

FDA approved uses

  • Respiratory tract infections caused by Streptococcus pneumoniae and Streptococcus pyogenes.
  • Otitis media due to Streptococcus pneumoniae, Haemophilus influenzae, Staphylococcus aureus, Streptococcus pyogenes, and Moraxella catarrhalis.
  • Skin and skin structure infections, including cellulitis, erysipelas, and impetigo, caused by Staphylococcus aureus or Streptococcus pyogenes.
  • Bone infections caused by Staphylococcus aureus or Proteus mirabilis.
  • Genitourinary tract infections, including cystitis, acute prostatitis, caused by Escherichia coli, Proteus mirabilis, and Klebsiella pneumoniae. (See Cephalexin for UTIs)


Spectrum of activity

  • Gram-positive Aerobes: Staphylococcus aureus, Streptococcus pneumoniae, Streptococcus pyogenes
  • Gram-negative Aerobes: Escherichia coli, Haemophilus influenzae, Klebsiella pneumoniae, Moraxella catarrhalis, Proteus mirabilis

Off-label uses

  • Mastitis, a breast inflammation often caused by a blocked milk duct that hasn't cleared.
  • Folliculitis
  • Prevention of infective endocarditis
  • Diverticulitis 5

Cephalexin "pros" and "cons"


  • Excellent safety record
  • Very low incidence of allergic reactions
  • May be taken without regard to food
  • Very high oral bioavailability
  • Superior to penicillin in the streptococcal throat infection 2.
  • Very effective for mild cases of cellulitis 3. Cephalexin remains the preferred antibiotic for the treatment of uncomplicated streptococcal cellulitis4.
  • Active against most gram-positive bacteria.


  • Short half-life requires QID dosing: the serum half-life is 0.5–1.2 hours in adults, about 5 hours in neonates and 2.5 hours in children 3–12 months of age.
  • Less active than cefatrizine and cefaclor against Escherichia coli, Klebsiella pneumoniae and Proteus mirabilis 2.

Mode of action

Cephalexin inhibits the third and last stage of bacterial cell wall synthesis by preferably binding to specific proteins (PBPs) that are located inside the bacterial cell wall.

Time for Cephalexin to clear out the system

Cephalexin mean half-life is 50 to 80 min. It may take 4 to 5 hours to clear out of the system.


Further reading


  • 1. Disney FA, Dillon H, Blumer JL, Dudding BA, McLinn SE, Nelson DB, Selbst SM. Am J Dis Child. 1992 Nov;146(11):1324-7. PubMed
  • 2. Luh KT, Chao HP, Ho SW, Hsieh WC. Comparison of in vitro antibacterial activity of four oral cephems. Zhonghua Min Guo Wei Sheng Wu Ji Mian Yi Xue Za Zhi. 1984 Feb;17(1):11-8. PubMed
  • 3. Pallin DJ, Binder WD, Allen MB, Lederman M, Parmar S, Filbin MR, Hooper DC, Camargo CA Jr. Effectiveness of cephalexin plus TMP-SMX versus cephalexin alone for treatment of uncomplicated cellulitis: a randomized controlled trial.. Clin Infect Dis. 2013 Jun;56(12):1754-62 PubMed
  • 4. Cunha BA. Cephalexin remains the preferred antibiotic for uncomplicated cellulitis. Journal of Chemotherapy. 2014 Apr;26(2):65-6.
  • 5. Kevin R. Loughlin, Joyce A. Generali, Eds. (2006) The Guide to Off-Label Prescription Drugs New York: Free Press, p.702

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


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