Table of ContentsView AllTable of ContentsWhat It IsUsesResistanceBefore TakingDirectionsSide EffectsWarnings
Table of ContentsView All
View All
Table of Contents
What It Is
Uses
Resistance
Before Taking
Directions
Side Effects
Warnings
SDI Productions / Getty Images
Penicillin is anantibioticused to treat certain types ofbacterial infections, such as pneumonia, meningitis, or strep throat.
There are different types of penicillin with differentmechanisms of action. All forms are derived, at least in part, from the fungusPenicillium chrysogenum.
This article discusses penicillin, its uses, and how it should be taken. It also goes over penicillin resistance and precautions you should take when using this medication.
What Is Penicillin?
Penicillins are antibiotics that belong to a larger family of drugs known as beta-lactam antibiotics. Penicillin was the world’s first widely used antibiotic.
Alexander Fleming discovered penicillin in 1928. Its discovery was considered accidental; Fleming noticed that fungus growing on an agar plate appeared to be inhibiting the growth of bacteria. He isolated the active agent in the fungus and named it penicillin.
Natural penicillins are those directly derived fromP. chrysogenumfungi. There are two natural penicillins.
Semisynthetic penicillins are produced in a lab to resemble chemical substances found inP. chrysogenum. Semisynthetic penicillins include commonly prescribed antibiotics like amoxicillin and ampicillin.
NaturalPenicillin G (benzylpenicillin)Penicillin V (phenoxymethylpenicillin)SemisyntheticAminopenicillins (ampicillin, amoxicillin, and hetacillin)Antistaphylococcal penicillins (cloxacillin, dicloxacillin, nafcillin, and oxacillin)Broad-spectrum penicillins (piperacillin)Beta-lactamase inhibitor (clavulanic acid)
NaturalPenicillin G (benzylpenicillin)Penicillin V (phenoxymethylpenicillin)
Penicillin G (benzylpenicillin)
Penicillin V (phenoxymethylpenicillin)
SemisyntheticAminopenicillins (ampicillin, amoxicillin, and hetacillin)Antistaphylococcal penicillins (cloxacillin, dicloxacillin, nafcillin, and oxacillin)Broad-spectrum penicillins (piperacillin)Beta-lactamase inhibitor (clavulanic acid)
Aminopenicillins (ampicillin, amoxicillin, and hetacillin)
Antistaphylococcal penicillins (cloxacillin, dicloxacillin, nafcillin, and oxacillin)
Broad-spectrum penicillins (piperacillin)
Beta-lactamase inhibitor (clavulanic acid)
Each of these types has a slightly different molecular structure and may be administered differently than the others.
Choosing the Right Antibiotic for a Bacterial Infection
What Does Penicillin Treat?
Penicillins are used for treating bacterial infections. They don’t treat viral, fungal, or parasitic infections. The drugs are generally active againstgram-positive bacteria, a group of bacteria that has peptidoglycan on the outside of the cell wall. With gram-negative bacteria, the peptidoglycan layer is buried beneath a layer of lipid cells, making it harder for the drug to access the molecule.
Natural penicillins—penicillin G and penicillin V—are still used today and are appropriate for the treatment of certain common and uncommon bacterial infections.
Semisynthetic antibiotics like amoxicillin—one of the most commonly prescribed antibiotics today—can be used to treat a broad spectrum of respiratory, skin, and bacterial infections likeH. pylori,Lyme disease, and pneumonia.
Are Amoxicillin and Augmentin the Same?
Off-Label
The off-label use of penicillins is common, albeit more often with drugs like amoxicillin and ampicillin than natural penicillins. Off-label use includes treatment of critical care patients withsepsisor newborns withacute respiratory distress. In neither instance are the drugs indicated for such use, but they are often considered necessary when no other treatment options are available.
Penicillin G is sometimes used off-label to treatprosthetic joint infectionsandLyme disease.Penicillin V is occasionally used off-label to treat Lyme disease andotitis media, or to prevent infections in people undergoingstem cell transplant.
Why You Don’t Need an Antibiotic to Treat Flu
Penicillin Resistance
Penicillin resistance has been known since the 1960s, when scientists began developing the first semisynthetic penicillin drugs to treat a broader range of bacterial infections. Penicillin resistance occurs when mutant bacterial strains resistant to the antibiotic are passed throughout a population.
Today, there is a growing number of bacterial infections that are either fully or partially resistant to the original penicillin drugs, includingNeisseria gonorrhoeae(gonorrhea) andmethicillin-resistantStaphylococcus aureus(MRSA).
Streptococcus pneumoniae(a type ofbacterial pneumonia) and certain types ofClostridioidesandListeriabacteria have become less responsive to these antibiotics as well.
The overuse of antibiotics in livestock to promote growth is known to increase the risk of resistant bacteria, includingsuperbugs, all along the food chain.As a result of this growing global concern, the United States banned the use of antibiotics for growth promotion in animals in 2017.
Penicillin can be very effective if used appropriately. Even so, there are instances when the drug is not effective in clearing an infection. In such cases, antibiotic susceptibility testing (also known as antibiotic sensitivity testing) may be used to determine if a person’s infection is responsive to penicillin.
Precautions and Contraindications
Penicillins are contraindicated if you’ve had a prior allergy to any drug in the penicillin family. It should also be used with extreme caution if you have ever had a severedrug hypersensitivity reactionin the past, includinganaphylaxis,Stevens-Johnson syndrome (SJS), or toxic epidermal necrosis (TEN).
If you have had an allergic reaction to penicillin G or penicillin V in the past, you may be—but are not necessarily—allergic to semisynthetic penicillins like amoxicillin or ampicillin.
Other beta-lactam antibiotics should be used with caution in people with penicillin allergy as there is a risk, albeit slight, of a cross-reactive allergy. This includescephalosporinantibiotics like cephalexin, cefepime, ceftriaxone, and Suprax (cefixime).
If you are concerned that you may be allergic to penicillin, you can haveskin allergy testingto see if you react to a minute amount of the drug placed under the skin.
Penicillin should also be used with extreme caution if you haveacute renal (kidney) failure. Penicillin is mainly excreted through the kidneys, and diminished kidney function can cause the drug to accumulate to toxic levels. The ensuing overdose of penicillin can lead to symptoms of agitation, confusion, stupor, abnormal twitches, and, in rare cases, coma.
The Facts About Penicillin and Cephalosporin Allergies
How to Take and Store Penicillin
Penicillin G
Penicillin G is available as either a premixed solution or a powder that is reconstituted with sterile water for injection. The premixed solution can be stored in the refrigerator or freezer, while the powder formulation can be kept safely at room temperature.
Penicillin G injections are not self-administered.
Penicillin V
Penicillin V is available as an oral tablet or a cherry-flavored powder mixed with water. Both can be safely stored at room temperature. Once the powder is reconstituted, it should be stored in the refrigerator and discarded after 14 days.
Penicillin V should be taken on an empty stomach to ensure maximum absorption. It should be taken at least one hour before a meal or at least two hours after a meal.
If you miss a dose of penicillin V, take it as soon as you remember. If it is near the time of your next dose, skip the dose and continue as normal. Never double up on doses.
Use as DirectedAlways take penicillin as directed and to completion. Do not stop because you feel well. You need to take the entire course so that all bacteria are eradicated. Small amounts of remaining bacteria can proliferate once treatment is stopped.
Use as Directed
Always take penicillin as directed and to completion. Do not stop because you feel well. You need to take the entire course so that all bacteria are eradicated. Small amounts of remaining bacteria can proliferate once treatment is stopped.
Why You Should Never Use Someone Else’s Antibiotics
Most penicillin side effects are mild and transient and will resolve on their own without treatment. But sometimes side effects can be severe—and even life-threatening—and require emergency care.
Common
The most common side effects of penicillins include:
Fever andangioedema(tissue swelling) can also occur but are less common.
Common and Serious Antibiotic Side Effects in Children
Severe
One of the most serious concerns associated with the use of penicillin is the risk of a potentially life-threatening, whole-body allergy calledanaphylaxis. True penicillin-induced anaphylaxis is rare, affecting about .015% to .04% of those taking penicillin.
Anaphylaxis can reap serious harm if it is left untreated. It can lead to shock, coma, respiratory or cardiac failure, and even death.
When to Call 911Seek emergency care if you experience any of thesymptoms of anaphylaxisafter receiving a dose of penicillin:Shortness of breathWheezingDizziness, lightheadedness, or faintingSevere rash or hivesRapid or irregular heartbeatSwelling of the face, tongue, or throatFeeling of impending doom
When to Call 911
Seek emergency care if you experience any of thesymptoms of anaphylaxisafter receiving a dose of penicillin:Shortness of breathWheezingDizziness, lightheadedness, or faintingSevere rash or hivesRapid or irregular heartbeatSwelling of the face, tongue, or throatFeeling of impending doom
Seek emergency care if you experience any of thesymptoms of anaphylaxisafter receiving a dose of penicillin:
Antibiotics Most Likely to Cause Diarrhea
Warnings and Interactions
Penicillins are generally considered safe during pregnancy and breastfeeding. Evidence in humans is lacking, but animal studies have shown no risk of fetal harm.
If you are pregnant, planning to get pregnant, or breastfeeding, speak with your healthcare provider to fully understand the benefits and risks of using penicillin.
Drugs that are likely to interact with penicillin include:
To avoid interactions, always let your healthcare provider know about any drugs you are taking, whether they are prescription, over-the-counter, nutritional, herbal, or recreational.
If you have kidney disease, you may need a lower penicillin dose to prevent drug toxicity.If you are treated withhemodialysis, however, you may need a higher dose because hemodialysis can speed the clearance of penicillin from the blood.
Summary
Some bacteria are becoming resistant to penicillin, so it is sometimes given in combination with other medicines. If you take penicillin, be sure to finish the entire course of medication even if you feel better. This can help prevent antibacterial resistance.
Some people are allergic to penicillin. Talk to your healthcare provider if you’ve previously had an allergic reaction to any drug in the penicillin family.
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Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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