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Generic Name: cephalexin

Brands: Daxbia, Keflex

Class: first generation semisynthetic cephalosporin antibacterial drug (cephalosporins)

Molecular Formula: C16H17N3O4S

Substance UNII: 5SFF1W6677

What is Cephalexin?

Cephalexin is part of a class of antibacterial drugs called cephalosporins; it is similar to another class of antibacterial drugs called penicillin. (also it has a similar chemical structure) Cephalexin is often sold under the brand names Daxbia and Keflex.

What is Cephalexin Used For?

Cephalexin is chiefly used to treat infections caused by bacteria. Common conditions this medication treats include include:

  • Ear infections (e.g., otitis media)
  • Skin infections
  • Bone infections
  • Upper respiratory infections (e.g., bacterial pneumonia, tonsillitis, laryngitis)
  • Genitourinary infections (e.g., acute prostatitis, urinary tract infection)

According to the U.S. National Library of Medicine, cephalexin is sometimes used for patients who have a heart condition and need a procedure or are about to undergo a dental or upper respiratory tract procedure involving the nose, throat, mouth, or voice box, but are allergic to penicillin. Taking this drug before their procedure may help prevent them from developing an infection in the heart or at surgical site.

How Does Cephalexin Work?

Cephalexin kills bacteria by preventing the formation of bacterial cell walls. Specifically, the drug binds to and blocks certain enzymes that normally help make a molecule in bacteria called peptidoglycan, which is an important part of the cell wall structure. This binding therefore causes bacterial cells to break down and die.

The drug is considered a broad-spectrum antibiotic because it is effective in fighting many common bacteria, including:

  • Streptococcus pneumoniae 
  • Streptococcus pyogenes
  • Haemophilus influenzae
  • Staphylococcus aureus
  • Moraxella catarrhalis 
  • Escherichia coli 
  • Proteus mirabilis
  • Klebsiella pneumoniae

Compared to other types of cephalosporins, cephalexin is more active against gram-positive (versus gram-negative) organisms. Now, gram-positive bacteria are bacteria that have no outer lipid (fatty) membrane but do possess a thick peptidoglycan layer; the inverse is true for gram-negative bacteria. Considering that this medication works by attacking peptidoglycan, it should come as no surprise that it is more effective against gram-positive bacteria.

NOTE: This antibiotic is not always effective against the above bacteria. Always follow your physicians instructions exactly whenever taking any antibiotic.

How Long Does it Take for Cephalexin to Work?

Cephalexin starts to work right away, and peak concentration in the body is achieved after one hour of taking it. That being said, it can take up to 48 hours after taking the drug for a person to see symptom improvement.

Do Not Use Cephalexin If:

There are several situations where this medication may not be the right choice for you. According to the U.S. Food and Drug Administration (FDA), a person should not use cephalexin if they have:

  • A history of an allergic reaction to cephalexin or related medications, including penicillin (NOTE: just because you are allergic to penicillins does not necessarily mean you will be allergic to cephalexin (but there may be increased chance due to similar chemical structure))
  • A history of severe bloody diarrhea after taking antibiotics
  • A history of a severe skin rash, blistering, or mouth sores after taking antibiotics
  • A medical condition requiring the use of blood thinners
  • Intestinal problems, like colitis (colon inflammation)
  • Kidney problems

Please note that this list may not be complete, and there may be other situations where cephalexin use is not advisable. 

Disclaimer: this article does not constitute or replace medical advice. If you have an emergency or a serious medical question, please contact a medical professional or call 911 immediately. To see our full medical disclaimer, visit our Terms of Use page.


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