What to Know About Penicillin G

An Antibiotic Drug Approved to Take on Bacterial Infection.

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Penicillin G (penicillin G potassium), is an antibiotic prescribed for the treatment of bacterial infections. It's used for treating pneumonia, strep throat, staph infection, diphtheria, meningitis, gonorrhea, and syphilis. It may be administered to prevent heart valve infection before dental procedures for people with certain cardiac conditions.

Part of a class of antibiotics called “natural penicillins,” Penicillin G acts directly on bacteria in the body and kills these pathogens by weakening their cell walls. Taken intravenously (IV) or as an injection, it’s also marketed under the name Pfizerpen.

Penicillin G injection
 microgen / iStock / Getty Images


An antibiotic that works against a wide range of bacteria, Penicillin G is effective against infections caused by some forms of Streptococcus pyogenes, Streptococcus pneumoniae, Staphylcoccus, Clostridium bacteria, and many others.

It is used for treating:

  • Septicemia and sepsis. Bacteria in the bloodstream
  • Pneumonia: A lung infection
  • Empyema: This respiratory infection of lung tissues often associated with pneumonia
  • Pericarditis: Infection of the pericardium (the tissues around the heart)
  • Endocarditis: An infection of the inner lining of the chambers and valves of the heart
  • Meningitis: An infection of the protective membranes that surround the brain and spinal cord
  • Anthrax: This infection of the skin arises due to bacteria called Bacillus anthracis
  • Actinomycosis: This is a rare infection of the skin, soft tissues, and, in extreme cases, the blood
  • Botulism: An illness caused by a toxin that's released by the Clostridium botulinum bacteria
  • Diphtheria: Life-threatening breathing problems, heart failure, or paralysis caused by the Corynebacterium diphtheriae
  • Erysipelothrix endocarditis: A rare infection of the inner chambers of the heart
  • Fusospirochetosis: Commonly known as “trench mouth,” this is a painful infection of the gums
  • Listeria infection: Primarily affecting newborns, pregnant people, older adults, and those with compromised immune systems, this infection can lead to symptoms ranging from fever and diarrhea to loss of balance, headache, and convulsions
  • Pasteurella infection: A skin and soft tissue infection due to animal bites and scratches
  • Haverhill fever: Sometimes called “rat bite fever,” this infection arises due to rodent bites or scratches
  • Gonorrhea: A sexually-transmitted disease (STD)
  • Syphilis: An STD caused by Treponema pallidum

Before Taking

Among the important considerations are the severity of the case as well as the specific type of bacteria involved. Following assessment of symptoms and medical history, healthcare providers may conduct several different tests to identify the nature and scope of the issue:

Stains and Microscopy

A specimen is taken from saliva, blood, or tissue for microscopic examination. This approach doesn’t provide definitive identification of the specific bacterial strain, but it can be used to quickly categorize the broader class of the pathogen at play.


Samples are taken from saliva, blood, or bodily tissue and placed in specialized broth or agar plates that allow them to survive and multiply. Further biochemical tests can lead to definitive identification. After initial identification, antibiotics may be applied to the sample to see if they are effective against the strain.

Dark Field and Fluorescent Microscopy

Dark field identification employs ultraviolet (UV) light to assess specimens through a microscope. A dark background is used, and the bacteria is viewed from the side rather than the back of the slide, allowing healthcare providers to examine the structure of the pathogen cells.

This may be combined with fluorescent microscopy, which is effective in identifying syphilis strains as well as tuberculosis strains.

Antigen Detection

Coming in the form of commercial kits, antigen tests are employed on bodily fluids to provide rapid results. Depending on the specific disease suspected, urine, throat swab, or cerebrospinal fluid, among others, may be used. 

Nucleic Acid Probes and Polymerase Chain Reaction

Nucleic acid probes are specialized molecules used to detect bacteria in bodily fluids or other samples. Polymerase chain reactions (PCR) aid in identification by generating copies of existing bacterial genetic material in a sample.


This approach involves assessing certain chemical interactions within the blood serum. The healthcare providers look for signs of a body’s immune response to bacteria, which can help pin down the specific infection at play.

Precautions and Contraindications

Once the strain of bacteria has been properly identified, the practitioner will need to weigh some other factors before prescribing therapy with penicillin G. As with any medication, some people may not be the best candidates for this approach, due to health status or other medications that they’re taking.

Here’s a quick breakdown of some of the factors that are considered:

  • Penicillin allergy: Allergic reaction to penicillin G can be quite severe, so let your healthcare provider know if you’re allergic to any type of penicillin or any antibiotic.
  • Antibiotic interactions: Other antibiotics can interfere with the efficacy of this injection. Adjustments in dosage may need to be made if you take cephalosporin antibiotics such as cefaclor, cefadroxil, Ancef (cefazolin), Spectracef (cefditoren), or Suprax (cefixime), among others. 
  • Certain medications/supplements: Some prescription or over-the-counter medications can interact with penicillin G. Among the substances that can influence dosage are aspirin, chloramphenicol, diuretics (water pills), as well as sulfa antibiotics, and Achromycin (tetracycline). 
  • Liver disease: The liver plays an important role in processing medications that you take, so your healthcare provider would take special consideration before prescribing penicillin G if you have liver damage.
  • Heart disease: Though this medication is sometimes administered for preventing infection if you have certain cardiac problems, if you have a history of heart disease, make sure to let your healthcare provider know.
  • Kidney disease: Like the liver, the kidneys are involved in cleaning out the blood and processing medications you’ve taken. It’s important to let your practitioner know if you currently have or have had kidney problems.
  • Nursing: Penicillin G can be passed via breastmilk, so you need to let your healthcare provider know if you are breastfeeding.
  • Pregnancy: While there’s no evidence of harm from fetal exposure to this drug, not enough studies have been performed to conclusively rule out harm.
  • Age: Newborns may not be able to properly metabolize penicillin G, so this medication isn’t often administered to this group. Children can take this drug, but dosage is adjusted based on weight (see below).

As a patient, it’s absolutely critical that you give your healthcare provider a full accounting of your medical history as well as a list of every prescription or over the counter medication, vitamin, or supplement that you’re taking. This way they’ll be better able to come up with a treatment that’s safe and effective.

Other Natural Penicillins

Penicillin G is part of a class of drugs called natural penicillins. These were the first antibiotic drugs developed and are derivations of naturally occurring compounds.

Others of the class include:

  • Bicillin L-A (penicillin G benzathine)
  • Penicillin VK (penicillin V potassium)
  • Beepen VK, Veetids (penicillin V potassium)
  • Bicillin C-R (penicillin G benzathine/procaine penicillin)
  • Bicillin C-R 900 / 300 (penicillin G benzathine/procaine penicillin)
  • Isoject Permapen (penicillin G benzathine)
  • Wycillin (procaine penicillin)


Your dose of penicillin G administered will depend on the specific disease you are being treated for, your health status, as well as a number of other factors. Typically, this drug is administered in divided doses, that are scheduled every four to six hours, though some conditions may require a different course. 

Some of the infections that are treated with penicillin G are fairly common, while some are quite rare.

Here’s a quick breakdown of recommended doses for adults based on condition:

  • Severe streptococcal infections: This class of diseases includes forms of pneumonia, septicemia, endocarditis, pericarditis, and meningitis. Divided doses of 12 to 24 million units per day, every four to six hours are recommended.
  • Severe staphylococcal infections: This bacteria can cause pneumonia, septicemia, empyema, pericarditis, meningitis, and endocarditis. Based on severity, dosages will vary from 5 to 24 million units a day, in equally divided doses, every four to six hours.
  • Anthrax: This serious skin infection is treated with a minimum of 8 million units every six hours, though larger amounts may be needed.
  • Actinomycosis: When this skin and tissue infection spreads to the neck and face, it causes a condition called cervicofacial disease, which is treated with 1 to 6 million units/day. If the infection spreads to the abdomen and thorax (the part of the torso below the neck and above the abdomen), the dose is increased to 10 to 20 million units a day. 
  • Clostridial infection: Diseases caused by Clostridia bacteria include botulism, gangrene, and tetanus. In these cases, 20 million units/day is recommended.   
  • Diphtheria: For the treatment of this infection, Penicillin G is usually administered alongside other therapies or as a means to prevent becoming a carrier. In this case, two to three million units/day in divided doses for 10-12 days is indicated.
  • Erysipelothrix endocarditis: This dangerous heart condition is treated with 12 to 20 million units/day for four to six weeks.
  • Fusospirochetosis: For severe cases of this gum infection that have begun to spread to other parts of the body, the recommended dosage is 5 to 10 million units per day.
  • Listeria infections: When infection with Listeria monocytogenes causes meningitis (affecting tissues around the brain and brainstem), a dosage of 15 to 20 million units/day for two weeks is indicated. In cases where the heart becomes involved, the same course of administration is extended to four weeks. 
  • Pasteurella infection: Infections of this bacteria are treated with 4 to 6 million units/day for two weeks.
  • Haverhill fever: Fevers and other diseases related to animal bites call for anywhere from 12 to 24 units daily for three to four weeks.
  • Gonoccocal infection: In cases where untreated gonorrhea has started to spread to other bodily systems, penicillin G is administered in doses of 10 million units/day, with the duration of therapy guided by the severity of the disease.
  • Syphilis: Most often employed once syphilis has started to spread to the brain (a condition called neurosyphilis), doses 12 to 24 million units/day split into two to four million units every four hours for 10-14 days.
  • Meningococcal infection: Meningitis or septicemia resulting from the bacteria, Neisseria meningitidis, is treated with 24 million units/day as 2 million units every 2 hours.

Remember that the specific dosages that you would be administered are based on these guidelines, but are ultimately up to your healthcare provider; what works for you might not for someone else.    


Dosage of penicillin G will need to be adjusted for pediatric cases and those with severe kidney problems. For children, the necessary amount of medication is calculated based on the child's weight, and if the recommended dose is less than 1 million units/day, your child's healthcare providers may consider alternatives.


Prescribing guidelines for children are as follows:

Serious infection with streptococcus/meningococcus: Pneumonia, endocarditis, and other severe infections by strains of streptococcal and meningococcal bacteria are treated with 150,000 to 300,000 units per kilogram (kg) body weight per day in equal doses every four to six hours (one kg is approximately 2.2 pounds). Duration will vary based on condition.

Meningitis due to pneumococcus/meningococcus: When these bacteria spread to the meninges, 250,000 units/kg/day is indicated in divided, equal doses every four hours for seven to 14 days, depending on condition. No more than 12 to 20 million units/day should be administered.

Spreading gonococcal infections: In children weighing less than 45 kg, the dosage will depend on what bodily system is infected.

  • If joints are attacked, leading to arthritis, 100,000 units/kg/day in four, equally divided doses for seven to 10 days is indicated.
  • Meningitis in these cases calls for 250,000 units/kg/day in equal doses every four hours for 10 to 14 days. This dosage is the same for endocarditis due to gonococcus bacteria, though the duration of treatment is extended to four weeks.
  • If these symptoms are present in children above 45 kg, 10 million units a day in four, divided doses is indicated, with duration depending on the disease.

Congenital syphilis: In children after the newborn period with syphilis (acquired from the parent during pregnancy), 200,000 to 300,000 units/kg/day in divided doses every four hours is indicated. Typically, this therapy lasts 10 to 14 days.

Diphtheria: As an adjunct therapy to antitoxins used to combat this infection and/or to prevent becoming a carrier, 150,000 to 250,000 units/kg/day in equal doses every six hours for seven to 10 days is recommended.

Haverhill/rate-bite fever: In these cases, the indicated dose is 150,000 to 250,000 units/kg/day in equal doses every four hours for four weeks.

Pre-Existing Kidney Disease

Another adjustment that needs to be made is in cases of severe kidney problems.

Creatinine clearance is a measure of renal function. If this figure is above 10 ml/min, it suggests underlying kidney disease, and full dosages (as indicated above) are bolstered by additional half dosages every four to five hours.

How to Take and Store

Penicillin G comes in a liquid form and is most often given either via IV or as a shot. This is often administered in the hospital, though, in some cases, you may be able to take it at home.

In both cases, your healthcare provider will have already determined the necessary dosage, so you’ll just need to follow any given schedule and procedures. Especially if you’re using this antibiotic at home, pay careful attention to your practitioner’s instructions about care and administration, and don’t hesitate to ask any questions you may have.

If you’ve been given this drug to take at home, you’ll likely receive it in one of two forms—as a powder that needs to be mixed or as a premixed solution.

What should you keep in mind? Here’s a quick breakdown:

  • Store frozen: It’s recommended that you store this medicine in the freezer at a temperature of at most minus 20 C (minus 4 F), especially in its pre-mixed, liquid form.
  • Thaw before use: Thaw solutions to room temperature in a plastic container prior to use. Once it’s warmed up, shake up the container a little. Never use a microwave or other means to warm up the solution, and don’t refreeze it.
  • Proper mixing: If you’ve received a powdered form that needs to be mixed with liquid before use, make sure you have a clear sense of how to do this properly.
  • Inspect packaging: Do not use penicillin G if you feel or see any leaks in the packaging or see signs of damage on the outlet port. Throw that dose out or take it to your pharmacy so you can get a replacement dose.
  • Storage of thawed solutions: Typically, already thawed solution can be stored in the refrigerator for up to 14 days.
  • Careful administration: Prepare for IV administration by suspending the container from its eyelet support, removing the protector from the outlet port on the bottom, and attach the set used for administration (being very careful to follow given instructions).
  • Kit care: Be very careful with the administration kit and make sure to follow all the instructions given. If you see any signs of tampering or damage, don’t use it.


What if you accidentally take too much? If you overdose with penicillin G, you may experience a range of symptoms, including agitation, confusion, hallucination, and seizures, among others. This is a medical emergency, so seek out prompt medical attention.

Finally, if you discover you’ve missed a dose, take a normal one as soon as you can. That said, if it’s almost time for your next dose, you can skip it and return to your schedule. Never try to double-up on these.

Side Effects

As with all medications, people taking penicillin G may be subject to a range of side effects, both common and more severe. Let your healthcare provider know about your side effects and get prompt medical attention if you experience severe symptoms. Some adverse reactions are medical emergencies.


If you’ve been prescribed penicillin G, there’s a chance that you may experience any of the following side effects:

  • Diarrhea
  • Nausea
  • Vomiting
  • Sore or irritation in the mouth
  • Change of color of the tongue
  • Irritation at injection site


By far the most severe adverse reaction to Penicillin G is severe allergic shock, which can be deadly.

Additionally, if you experience any of the below symptoms, get medical attention immediately:

  • Rash
  • Hives
  • Itching
  • Red, swollen, blistered, or peeling skin
  • Fever
  • Wheezing
  • Tightness in the chest or throat
  • Breathing problems
  • Trouble swallowing/talking
  • Hoarse voice
  • Swelling in the mouth, face, lips, tongue, or throat 

In addition, seek immediate attention if you have any of the following:

  • Irregular or changed heartbeat
  • Difficulties with thinking logically
  • Weakness
  • Lightheadedness/dizziness
  • Numbness or tingling
  • Shortness of bread
  • Sores in the mouth, throat, nose, or eyes
  • Skin reactions such as red, swollen blistered skin
  • Red/irritated eyes
  • Fever
  • Chills
  • Sore throat
  • Rapid heartbeat
  • Fast breathing
  • Yellowing of skin or eyes
  • Muscular/joint pain
  • Stomach pain
  • Seizures
  • Difficulty urinating
  • Severe diarrhea with bloody stools, stomach pain, cramps

Warnings and Interactions

As mentioned, the most severe adverse reaction to Penicillin G is allergic shock, which can be deadly. These reactions can be immediate, with symptoms arising within 20 minutes of administration, or you can have a delayed reaction within one to two weeks.

The efficacy of this drug can also be affected by the presence of other drugs or chemicals in your system. In fact, many medications and substances are known to interact with this drug in some way. The most common of these are:

  • Acetylsalicylic acid (aspirin)
  • Activated charcoal (charcoal)
  • Adrenalin (epinephrine)
  • Albenza (albendazole)
  • B complex 100 (multivitamin)
  • Calcium 600 D (calcium / vitamin D)
  • Caltrate 600+D (calcium / vitamin D)
  • Chloromycetin (chloramphenicol)
  • Cyanoject (cyanocobalamin)
  • Cymbalta (duloxetine)
  • Digox (digoxin)
  • Folinic-Plus (multivitamin)
  • Hydrocortone (hydrocortisone)
  • Lasix (furosemide)
  • Levothyrox (levothyroxine)
  • Methadose (methadone)
  • Paracetamol (acetaminophen)
  • Phenytoin sodium (phenytoin)
  • Synthroid (levothyroxine)
  • Vitamin D3 (cholecalciferol)

The good news is that there are no dietary restrictions while taking this drug, and adults can responsibly enjoy alcohol. To ensure full efficacy of this treatment, make sure to stick to your prescription as best you can; don’t stop taking penicillin G without your healthcare provider’s approval, and keep them informed on your progress.

5 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. National Library of Medicine, National Center for Biotechnology Information. Penicillin G: Compound summary.

  2. U.S. Food and Drug Administration. Penicillin G potassium.

  3. Doron S, Gorbach S. Bacterial Infections: overviewInternational Encyclopedia of Public Health. 2008:273-282.

  4. Prescribers Digital Reference. Penicillin G potasssium—drug summary.

  5. MedlinePlus. Penicillin G (potassium, sodium).

By Mark Gurarie
Mark Gurarie is a freelance writer, editor, and adjunct lecturer of writing composition at George Washington University.