What to Know About Penicillin G

An Antibiotic Drug Approved to Take on Bacterial Infection.

In This Article

Penicillin G (penicillin G potassium), is an antibiotic most often prescribed to take on more severe bacterial infections. It's used for pneumonia, strep throat, staph infection, diphtheria, meningitis, gonorrhea, and syphilis. It may be administered to prevent heart valve infection when those with certain cardiac conditions require dental care or surgery.

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 some forms of Streptococcus pyogenes, Streptococcus pneumoniae, Staphylcoccus, Clostridium bacteria, as well as many others. As such, it may be indicated to take on more severe cases of a number of diseases, including:

  • Septicemia: Bacteria in the bloodstream create a serious and dangerous condition, which may lead to the release of toxins and sepsis.
  • Pneumonia: A severe, potentially life-threatening lung condition, cases arising due to bacterial infection can be treated with Penicillin G.
  • Empyema: This respiratory infection of lung tissues is often associated with pneumonia.
  • Pericarditis: This swelling of the pericardium—the tissues around the heart—can result from several bacterial strains that penicillin G works against.
  • Endocarditis: An infection of the inner lining of the chambers and valves of the heart, endocarditis is often susceptible to IV therapy with this antibiotic.
  • Meningitis: This inflammation of the meninges—the protective membranes that surround the brain and spinal cord structures can also arise due to bacteria treatable with this medicine.
  • Anthrax: This infection of the skin arises due to bacteria called, Bacillus anthracis, which penicillin G targets.
  • Actinomycosis: This is a rare infection of the skin, soft tissues, and, in extreme cases, the blood.
  • Botulism: Along with gangrene, and tetanus, botulism, a rare poisoning caused by the Clostridium botulinum bacteria, can be treated with this antibiotic.  
  • Diphtheria: Leading to breathing problems, heart failure, paralysis, and even death, diphtheria is caused by the Corynebacterium diphtheriae.  
  • Erysipelothrix endocarditis: This very rare infection can spread to the inner chambers of the heart, leading to severe symptoms.
  • Fusospirochetosis: Commonly known as “trench mouth,” this is a painful infection of the gums.
  • Listeria infection: Primarily affecting newborns, pregnant women, seniors, and those with compromised immune systems, Listeria infection can lead to symptoms ranging from fever and diarrhea to loss of balance, headache, and convulsions.
  • Pasteurella infection: This infection of skin and soft tissue results from animal bites and scratches.
  • Haverhill fever: Sometimes called “rat bite fever,” this infection arises due to rodent bites or scratches.
  • Gonorrhea: If this sexually-transmitted disease has spread, penicillin G may be indicated to take on the issue.
  • Syphilis: Penicillin G also takes on the bacteria Treponema pallidum, which is a cause of the sexually-transmitted disease, syphilis.

Before Taking

Penicillin G tends to be reserved for more severe cases of bacterial infection. 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, doctors may conduct several different tests to identify the nature and scope of the issue:

Stains and Microscopy

While this approach doesn’t allow for a definitive identification of the specific bacterial strain, it is able to quickly categorize the broader class of the pathogen at play.

Basically, a specimen taken from saliva, blood, or tissue is placed on a slide, where it’s stained with a pigment. When viewed under a microscope, the color, shape, and morphology of the bacteria, will help the lab professional determine what type they are, which will factor into what antibiotic will be recommended.


Employing samples taken from saliva, blood, or bodily tissue, this test can be particularly effective in identifying a number of bacterial strains. The samples are placed in a specialized broth or agar plates that allow them to survive and develop.

Based on the formation and spread observed, lab professionals will get a better sense of the bacteria involved. Further biochemical tests can lead to definitive identification. After initial identification, antibiotics may be applied 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 doctors 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. These also can be employed to rule others out.

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. For instance, the bacteria, E. coli, can be identified with a fecal sample, while evidence of gonorrhea or chlamydia will be seen in urine.

Polymerase chain reactions (PCR) aid in identification by generating copies of existing bacterial genetic material in a sample.


This approach, which involves assessing for certain chemical interactions within the blood serum, is typically employed in cases where culturing or other approaches may not yield results. Basically, via blood sample, the doctors are looking 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 doctor will need to weigh some other factors before prescribing therapy with penicillin G. As with any medication, some patients 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 factors that are considered:

  • Penicillin allergy: Allergic reaction to penicillin G can be quite severe, so let your doctor know if you know you’re allergic to any type of penicillin.
  • Allergy to other antibiotics: Other antibiotics can also hinder 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 prescribed or over-the-counter medications can also 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). 
  • Asthma: People with asthma may not benefit from this medication. Tell your doctor if this is your case so that they consider alternatives.
  • Liver disease: The liver plays an important role in processing medications that you take, so those with liver damage require special consideration.
  • Heart disease: Though sometimes administered to take on some cardiac problems, if you have a history of heart disease, make sure to let your doctor 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 doctor know if you currently have or have had kidney problems.
  • Nursing: Penicillin G can be passed via breastmilk, so you may be advised not to do this for some time while on this medication. Let your doctor know if you breastfeed or planning to start doing so.
  • Pregnancy: While there’s no evidence of harm to the unborn baby due to exposure to this drug, not enough studies have been performed to conclusively rule out harm. As such, doctors may also advise patients not to get pregnant during the course of treatment. 
  • Age: Newborns may not be able to properly 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 doctor a full accounting of your medical history as well as what 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 (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)
  • Veetids (penicillin V potassium)
  • Wycillin (procaine penicillin)


As with all antibiotics, the specific amount of penicillin G administered will depend on the specific disease being treated, the health status of the patient, as well as a number of other factors. Typically, this drug is administered in divided doses, meaning one dose will be split up and administered every four to six hours, though some conditions may require a different course. 

Here’s a quick breakdown of recommended doses for adult patients 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: Infections by this bacteria can also lead to 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 skin infection is treated with a minimum of 8 million units every six hours, though larger amounts may be needed.
  • Actinomycosis: If this skin and tissue infection has spread to the neck and face, a condition called cervicofacial disease, you’re given 1 to 6 million units/day. If this has spread 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: In cases of this disease, 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 becomes 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 length of therapy depending on severity of 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 specific dosages are up to your doctor; what works for you might not for someone else.    


Dosage of penicillin G will need to be adjusted for pediatric patients and those with severe kidney problems. In the former case, the necessary amount of medication depends on weight, and if less than 1 million units/day are recommended, doctors will explore alternatives. Prescribing guidelines for this population 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) 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, 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 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 mother 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.

Another adjustment that needs to be made is in cases of severe kidney problems. These organs are essential in removing waste from the blood and balancing fluid levels. While Penicillin G is relatively non-toxic, it can cause problems if organ function is affected.

Creatinine clearance is a measure of renal function; the lower the amount of creatinine, a waste product produced by muscles, the healthier these organs are. In cases of lower creatinine clearance—defined as less than 10 milliliters (ml) per minute—dosage is normal.

However, if this figure is above 10 ml/min, full dosages (as indicated above) are bolstered by additional half dosages every four to five hours. This treatment lasts 48 to 72 hours, with some cases calling for preventative therapy for streptococcal infections for seven to 10 days.

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 doctor 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 doctor’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 one of two forms—as a powder that needs to be mixed or as 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 solution and don’t refreeze.
  • 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.
  • 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 call for help and let your doctor know as soon as possible.

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, patients taking penicillin G may be subject to a range of side effects, both common and more severe. Let your doctor know if side effects are longstanding and disruptive, or if you see any signs of 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. If you experience any of the below symptoms, get help 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 there can be a more delayed reaction seen within one to two weeks (see above).

However, the efficacy of this drug can also be affected by the presence of other drugs or chemicals in the 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 for many patients 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 doctor’s approval and keep them informed on your progress.

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Article Sources
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