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Keflex vs. Penicillin

What’s the Difference Between Keflex and Penicillin?

What are Keflex and Penicillin?

Keflex (cephalexin) belongs to a class of antibiotics called cephalosporins. They are similar to penicillin in action and side effects. They stop or slow the growth of bacterial cells by preventing bacteria from forming the cell wall that surrounds each cell. The cell wall protects bacteria from the external environment and keeps the contents of the cell together, and without a cell wall, bacteria are not able to survive. Bacteria that are susceptible to cephalexin include Staphylococcus aureus, Streptococcus pneumoniae, Haemophilus influenzae, E. coli and several others.

Penicillins are antibiotics used to treat bacterial infections that are derived from the antibiotic penicillin. Penicillin antibiotics are used to treat many types of infections caused by susceptible bacteria. They are used to treat infections of the middle ear, sinuses, stomach and intestines bladder, and kidney. They also are used for treating pneumonia, blood infections (sepsis), uncomplicated gonorrhea, meningitis, endocarditis, and other serious infections. Penicillin itself is active against streptococci (including Streptococcus pneumoniae), Listeria, Neisseria gonorrhoeae, Clostridium, Peptococcus, and Peptostreptococcus.

What Are the Side Effects of Keflex vs. Penecillin?


The most common side effects of cephalexin are:

Individuals who are allergic to penicillin may also be allergic to cephalexin. Serious but rare reactions include seizures, severe allergic reactions (anaphylaxis), and low platelet or red blood cell count.

Cephalexin, like almost all antibiotics, may cause mild or severe cases of pseudomembranous colitis, a mild to severe inflammation of the colon. Antibiotics, including cephalexin alter the types of bacteria in the colon and permit overgrowth of a bacterium called Clostridium difficile. Studies indicate that toxins produced by Clostridium difficile are a primary cause of pseudomembranous colitis.


Side effects of penicillin antibiotics include

Individuals who are allergic to the cephalosporin class of antibiotics, which are related to the penicillins, for example, cefaclor (Ceclor), cephalexin (Keflex), and cefprozil (Cefzil), may or may not be allergic to penicillins.

Serious but rare reactions include

Like other antibiotics, penicillin antibiotics can alter the normal bacteria in the colon and encourage overgrowth of some bacteria such as Clostridium difficile, which causes inflammation of the colon (C. difficile colitis or pseudomembranous colitis).

Signs and symptoms of C. difficile colitis include

  • • diarrhea,
  • • fever,
  • • abdominal pain, and
  • • possibly shock.

What Is the Dosage for Keflex vs. Penicillin?


  • The dose of cephalexin for adults is 1 to 4 grams in divided doses.
  • The usual adult dose is 250 mg every 6 hours.
  • Some infections may be treated with 500 mg every 12 hours.
  • Children are treated with 25-100 mg/kg/day in divided doses.
  • The dosing interval may be every 6 or 12 hours depending on the type and seriousness of the infection.


Penicillin dosage is based on the disease type and severity. Penicillin antibiotics are available as

  • tablets,
  • capsules,
  • powder for oral suspension, and
  • powder for injection.


Bowel regularity means a bowel movement every day.
See Answer

What Drugs Interact with Keflex and Penicillin?


Cephalexin may reduce the effect of BCG and typhoid vaccines. Cephalexin should not be combined with BCG or typhoid vaccine unless there are no other options.


Penicillin antibiotics have few important drug interactions.

  • Probenecid (Benemid) causes an increase in the amount of penicillins in the body by preventing excretion of penicillin by the kidneys.
  • Combining ampicillin with allopurinol (Zyloprim) can increase the incidence of drug-related skin rash.
  • Penicillin antibiotics may reduce the effect of BCG live vaccine and typhoid live vaccine.

Are Keflex and Penicillin Safe to Use While Pregnant or Breastfeeding?


Cephalexin is excreted in breast milk. Cephalexin should be used with caution or stopped when breastfeeding.


Penicillin antibiotics are considered safe to use during pregnancy. Although small amounts of penicillins pass into breast milk they are considered safe to use while breastfeeding.


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