Penicillins are a group of closely related antibiotics that kill bacteria.


There are several types of penicillins, each used to treat different kinds of infections, such as skin infections, dental infections, ear infections, respiratory tract infections, urinary tract infections, gonorrhea, and other infections caused by bacteria. These drugs will not work for olds, flu, and other infections caused by viruses.

Examples of penicillins are penicillin V (Beepen-VK, Pen-Vee K, V-cillin K, Veetids) and amoxicillin (Amoxil, Polymox, Trimox, Wymox). Penicillins are sometimes combined with other ingredients called beta-lactamase inhibitors, which protect the penicillin from bacterial enzymes that may destroy it before it can do its work. The drug Augmentin, for example, contains a combination of amoxicillin and a beta-lactamase inhibitor, clavulanic acid. Penicillins are available only with a prescription.

The original form of penicillin is called penicillin G. It is a narrow-spectrum antibiotic, which can be destroyed by stomach acid, but it is still useful against anaerobic bacteria (bacteria that can live in the absence of air). Newer penicillins are resistant to stomach acid, such as penicillin V, or have a broader spectrum, such as ampicillin and amoxicillin.

General use

Penicillins are useful against infections in many parts of the body, including the mouth and throat, skin and soft tissue, tonsils, heart, lungs, and ears. However, since many bacteria are resistant to penicillin, it is often wise to do a culture and sensitivity test before using penicillins. In some cases, there are only a few types of bacteria that are likely to be a problem, and so it is appropriate to use a penicillin without testing. For example, dentists often prescribe penicillin to prevent infections after dental surgery.


Penicillins are usually very safe. The greatest risk is an allergic reaction, which can be severe. People who have been allergic to cephalosporins are likely to be allergic to penicillins. Moreover, people with certain medical conditions or who are taking certain other medicines can have problems if they take penicillins. Before taking these drugs, patients should be sure to let the physician know about any of the following conditions.

Low-sodium diet

Some penicillin medicines contain large enough amounts of sodium to cause problems for people on low-sodium diets. Parents of children on on such a diet should make sure that the physician treating the infection knows about the special diet.


Penicillins may cause false positive results on urine sugar tests for diabetes. People with diabetes should check with their physicians to see if they need to change their diet or the doses of their diabetes medicine.


Some formulations of Augmentin contain phenylalanine. People with phenylketonuria (PKU) should consult a physician before taking this medicine.

Side effects

The most common side effect of penicillin is diarrhea . Nausea , vomiting , and upset stomach are also common. With some penicillins, particularly the broad spectrum products, there is a risk of increased growth of organisms that are not affected by penicillin. This situation can lead to candidal infections of the mouth and vagina.

Most side effects of penicillin cannot be prevented. Amoxicillin has a lower incidence of diarrhea than ampicillin and is the preferred drug in most cases.


Birth control pills may not work properly when taken at the same time as penicillin. Penicillins may also interact with many other medicines. When this happens, the effects of one or both of the drugs may change or the risk of side effects may be greater. People who take penicillin should let their physician know all other medicines they are taking. Among the drugs that may interact with penicillins are the following:

  • acetaminophen (Tylenol) and other medicines that relieve pain and inflammation
  • medicine for overactive thyroid
  • other antibiotics
  • blood thinners
  • antiseizure medicines such as Depakote and Depakene
  • blood pressure drugs such as Capoten, Monopril, and Lotensin

The list above does not include every drug that may interact with penicillins. A physician or pharmacist should be consulted before a patient combines penicillins with any other prescription or nonprescription (over-the-counter) medicine.

Parental concerns

Parents should verify that their children have an infection requiring antibiotic therapy. Unnecessary use of antibiotics leads to development of bacterial resistance, while it subjects the child to some needless risk of adverse effects and wastes money.

Liquid forms of penicillin should be refrigerated after reconstitution. These preparations must be shaken well before use and measured with a medicinal teaspoon, not a household teaspoon.

Any adverse effects should be discussed with the prescriber. Penicillin should not be used in patients allergic to the drug; however, an incorrect report of an allergy to penicillin may cause prescribers to select a different drug which may cause even more severe side effects.

Penicillins should be administered exactly as directed. Users should never give larger, smaller, more frequent, or less frequent doses. To make sure the infection clears up completely, patients should take the medicine for as long as it has been prescribed. They should not stop taking the drug just because symptoms begin to improve. This point is important with all types of infections, but it is especially important with strep infections, which can lead to serious heart problems if they are not cleared up completely.

This medicine should be used only for the infection for which it was prescribed. Different kinds of penicillins cannot be substituted for one another. Do not save some of the medicine to use on future infections. It may not be the right treatment for other kinds of infections, even if the symptoms are the same.


Anaerobic —An organism that grows and thrives in an oxygen-free environment.

Beta-lactamase —An enzyme produced by some bacteria that destroys penicillins.

Broad spectrum —A term applied to antibiotics to indicate that they are effective against many different types of bacteria.

Enzyme —A protein that catalyzes a biochemical reaction without changing its own structure or function.

Microorganism —An organism that is too small to be seen with the naked eye, such as a bacterium, virus, or fungus.

Mononucleosis —An infection, caused by the Epstein-Barr virus, that causes swelling of lymph nodes, spleen, and liver, usually accompanied by extremely sore throat, fever, headache, and intense long-lasting fatigue. Also called infectious mononucleosis.



Beers, Mark. H., and Robert Berkow, eds. The Merck Manual , 2nd home ed. West Point, PA: Merck & Co., 2004.

Mcevoy, Gerald K., et al. AHFS Drug Information 2004 . Bethesda, MD: American Society of Healthsystems Pharmacists, 2004.

Siberry, George K., and Robert Iannone, eds. The Harriet Lane Handbook, 15th ed. Philadelphia, PA: Mosby Publishing, 2000.


Apter Andrea J., et al. "Represcription of penicillin after allergic-like events." Journal of Allergy and Clinical Immunology 113, no. 4 (April 2004): 764–770.


American Academy of Pediatrics. 141 Northwest Point Boulevard, Elk Grove Village, IL 60007–1098. Web site:

Centers for Disease Control. 200 Independence Avenue, SW, Washington, DC, 20201. Web site:


"Penicillins (Systemic)." Available online at (accessed September 29, 2004).

"Treat Sore Throat without Penicillin." Available online at (accessed September 29, 2004).

Nancy Ross-Flanigan Samuel Uretsky, PharmD

User Contributions:

Ananda Grobler
Mother & 2 daughters show the same response when using augmentin: nausea, throwing up and sleepiness, feeling tired. Somebody mentioned it might be an enzyme (or shortage?) in the liver of all three that might be the cause. Did you ever heard of such a cause.?

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