Penicillin (Antibiotics)

Medically Reviewed on 7/28/2022

What are penicillin antibiotics, and what are they used for?

Penicillins are antibiotics used to treat bacterial infections that are derived from the antibiotic penicillin.

Penicillin antibiotics stop bacteria from multiplying by preventing bacteria from forming the walls that surround them. The walls are necessary to protect the bacteria from their environment, and to keep the contents of the bacterial cell together. Bacteria cannot survive without a cell wall. Penicillin antibiotics are most effective when bacteria are actively multiplying and forming cell walls.

Today, many derivatives of penicillin have been developed that inhibit more types of bacteria than the original life-saving drug. Penicillin itself is active against

  • streptococci (including Streptococcus pneumoniae),
  • Listeria, Neisseria gonorrhoeae,
  • Clostridium,
  • Peptococcus, and
  • Peptostreptococcus.

However, most staphylococci now are resistant to penicillin.

Other penicillin antibiotics are effective against

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

In 1928, Alexander Fleming noted that mold belonging to the genus Penicillium inhibited the growth of bacteria. Fleming called this unknown antibacterial substance penicillin. Ten years later, a group at Oxford University began to investigate penicillin in laboratory mice. Penicillin was hailed as a miracle drug and saved countless lives in World War II.

What are examples of penicillin antibiotics available in the U.S.?

  • penicillin V
  • penicillin G (Pfizerpen, Permapen)
  • amoxicillin (Amoxil)
  • amoxicillin/clavulonate (Augmentin)
  • ampicillin (Unasyn)
  • nafcillin (Nallpen)
  • oxacillin (Bactocill)
  • dicloxacillin (Dycill, Dynapen are discontinued brands in the US; generic is available)
  • cloxacillin (discontinued in the US)
  • piperacillin (Pipracil)
  • piperacillin/tazobactam (Zosyn)
  • ticarcillin (Ticar) (Discontinued in the US; ; generic is not available))
  • ticarcillin/clavulonate (Timentin) (Discontinued in the US and a generic is not available.)


Bowel regularity means a bowel movement every day. See Answer

What are the side effects of penicillin antibiotics?

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

What drugs interact with penicillin antibiotics?

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 penicillin antibiotics safe to take during pregnancy or while 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.

What formulations of penicillin antibiotics are available?

Penicillin antibiotics are available as

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


Penicillin antibiotics are prescribed to treat a variety of types of infections. For example, middle ear and sinus infections; bladder, stomach, intestines, and kidney; pneumonia; sepsis; meningitis; endocarditis; and many other serious infections. Examples of penicillin antibiotics, side effects, drug interactions, and pregnancy and breastfeeding safety information should be reviewed prior to taking any medication.

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Medically Reviewed on 7/28/2022
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