What Is Penicillin Used For?

Penicillin is an antibiotic used to treat certain types of bacterial infections, such as pneumonia, meningitis, or strep throat.

There are different types of penicillin with different mechanisms of action. All forms are derived, at least in part, from the fungus Penicillium chrysogenum.

Penicillin may be administered by mouth or by injection. When given by injection, it may be administered intravenously (IV, into a vein) or intramuscularly (IM, in a large muscle).

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.

Penicillins work by binding to molecules on the walls of bacteria called peptidoglycan. When the bacteria divide, penicillin prevents proteins in the cell wall from reassembling properly, causing the bacterial cell to rupture and quickly die.

Natural penicillins are those directly derived from P. chrysogenum fungi. There are two natural penicillins.

Semisynthetic penicillins are produced in a lab to resemble chemical substances found in P. chrysogenum. Semisynthetic penicillins include commonly prescribed antibiotics like amoxicillin and ampicillin.

Each of these types has a slightly different molecular structure and may be administered differently than the others.

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 against gram-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.

The list of gram-positive bacteria that are treatable by penicillins includes those of the Clostridioides, Listeria, Neisseria, and Streptococcus genera.

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 like H. pylori, Lyme disease, and pneumonia.

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 with sepsis or newborns with acute 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 treat prosthetic joint infections and Lyme disease. Penicillin V is occasionally used off-label to treat Lyme disease and otitis media, or to prevent infections in people undergoing stem cell transplant.

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.

Streptococcus pneumoniae (a type of bacterial pneumonia) and certain types of Clostridioides and Listeria bacteria 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, including superbugs, 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.

To help overcome penicillin resistance, penicillins that don't have direct antibacterial activity may be used in combination therapies. For example, clavulanic acid blocks an enzyme secreted by antibiotic-resistant bacteria (beta-lactamase) that inhibits the activity of beta-lactam antibiotics.

Before Taking

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.

The test starts by culturing bacteria taken from a swab of body fluid, then directly exposing the bacteria to various penicillin types in a lab. Antibiotic susceptibility testing is often used for people with community-acquired pneumonia who are at high risk of severe illness or death.

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 severe drug hypersensitivity reaction in the past, including anaphylaxis, Stevens-Johnson syndrome (SJS), or toxic epidermal necrosis (TEN).

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 includes cephalosporin antibiotics like Keflex (cephalexin), Maxipime (cefepime), Rocephin (ceftriaxone), and Suprax (cefixime).

If you are concerned that you may be allergic to penicillin, you can have skin allergy testing to 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 have acute 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.

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.

Side Effects

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:

• Diarrhea
• Headache
• Stomach upset
• Nausea or vomiting
• Rash or hives (usually mild to moderate)
• Injection site pain (with penicillin G)
• Black hairy tongue
• Muscle twitches
• Oral thrush
• Vaginal yeast infection

Fever and angioedema (tissue swelling) can also occur but are less common.

Severe

One of the most serious concerns associated with the use of penicillin is the risk of a potentially life-threatening, whole-body allergy called anaphylaxis. 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.

On rare occasions, penicillins can cause acute interstitial nephritis, an inflammatory kidney condition most often caused by an abnormal immune reaction to medications. Symptoms include nausea, rash, fever, drowsiness, diminished urine output, fluid retention, and vomiting. Most cases are mild, but some can turn serious and cause an acute kidney injury.

Penicillins, like all antibiotics, are associated with an increased risk of Clostridioides difficile diarrhea. This is caused when bacteria that are normally present in the gut are obliterated by antibiotics, allowing C. difficile bacteria to proliferate. Most cases are mild and readily treatable, but C. difficile has been known on rare occasions to cause severe fulminant colitis, toxic megacolon, and death.

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.

A number of drugs can also interact with penicillin, oftentimes by competing for clearance in the kidneys. This can increase penicillin concentrations in the blood as well as the risk of side effects and drug toxicity. Other medications can speed the clearance of penicillin from the body and reduce the drug's effectiveness.

Drugs that are likely to interact with penicillin include:

• Anticoagulants (blood thinners) like Coumadin (warfarin)
• Diuretics (water pills) like Lasix (furosemide) and Edecrin (ethacrynic acid)
• Nonsteroidal anti-inflammatory drugs like aspirin, Tivorbex (indomethacin), and phenylbutazone
• Sulfonamides, like Bactrim (sulfamethoxazole/trimethoprim), Azulfidine (sulfasalazine), and Truxazole (sulfisoxazole)

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 with hemodialysis, however, you may need a higher dose because hemodialysis can speed the clearance of penicillin from the blood.

Summary

Penicillin is an antibiotic used to treat bacterial infections. It works by preventing bacteria from dividing properly. Penicillin is available by prescription and can treat gram-positive bacteria that cause illnesses such as strep throat, tetanus, and tonsillitis.

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.