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Pfizerpen Side Effects Center

Medical Editor: John P. Cunha, DO, FACOEP

Last reviewed on RxList 1/8/2016

Pfizerpen (penicillin G potassium) for Injection is an antibiotic used to treat severe infections including strep and staph infections, diphtheria, meningitis, gonorrhea, and syphilis. Pfizerpen is available in generic form. Common side effects of Pfizerpen include twitching or muscle spasm, skin rash, injection site reactions (pain, redness, or swelling), diarrhea, nausea, vomiting, headache, thrush (oral candidiasis), or vaginal yeast infection.

Dosage of Pfizerpen is dependent upon what it is being used to treat. Talk to your doctor about you individual dosage recommendation. The following drugs may interact with Pfizerpen: Adoxa, Doryx, Vibramycin, Periostat, Minocin, Broadspec, Panmycin, Sumycin, Bactrim, Cotrim, Gantanol, Septra, Sulfatrim, Gantrisin, Zithromax, and Dynabac. Tell your doctor all medications you are taking. Do not take Pfizerpen if you are allergic to aspirin. Pfizerpen will not treat a viral infection such as the common cold or flu. If you are pregnant, only take Pfizerpen if clearly needed. Exercise caution if you are breastfeeding while taking Pfizerpen.

Our Pfizerpen (penicillin G potassium) for Injection Side Effects Drug Center provides a comprehensive view of available drug information on the potential side effects when taking this medication.

This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.

What is Patient Information Overview?

A concise overview of the drug for the patient or caregiver from First DataBank.

Pfizerpen Overview - Patient Information: Side Effects

SIDE EFFECTS: Pain/redness/swelling at injection site may occur. If any of these effects persist or worsen, tell your doctor or pharmacist promptly.

Remember that your doctor has prescribed this medication because he or she has judged that the benefit to you is greater than the risk of side effects. Many people using this medication do not have serious side effects.

Tell your doctor right away if you have any serious side effects, including: joint/muscle pain, muscle weakness, swelling of ankles/feet, dark/cloudy urine, extreme tiredness, fast/slow/irregular heartbeat, new signs of infection (e.g., fever, persistent sore throat), easy bruising/bleeding, change in the amount of urine, numbness/tingling of arms/legs, seizures, uncontrolled movements, confusion.

If used to treat syphilis or other spirochetal infections (e.g., Lyme disease), this medication may cause a condition known as Jarisch-Herxheimer reaction. This condition may occur 1 to 2 hours after you receive the medication and can last up to 24 hours. Notify your doctor immediately if you experience any of these symptoms: fever/chills, muscle pain, worsening of skin sores, fast heartbeat, fast breathing, dizziness, flushing.

This medication may rarely cause a severe intestinal condition (Clostridium difficile-associated diarrhea) due to a type of resistant bacteria. This condition may occur weeks to months after treatment has stopped. Do not use anti-diarrhea products or narcotic pain medications if you have any of these symptoms because these products may make them worse. Tell your doctor immediately if you develop: persistent diarrhea, abdominal or stomach pain/cramping, blood/mucus in your stool.

Use of this medication for prolonged or repeated periods may result in oral thrush or a new vaginal yeast infection. Contact your doctor if you notice white patches in your mouth, a change in vaginal discharge, or other new symptoms.

A very serious allergic reaction to this drug is rare. However, get medical help right away if you notice any symptoms of a serious allergic reaction, including: rash, itching/swelling (especially of the face/tongue/throat), severe dizziness, trouble breathing.

This is not a complete list of possible side effects. If you notice other effects not listed above, contact your doctor or pharmacist.

In the US -

Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.

In Canada - Call your doctor for medical advice about side effects. You may report side effects to Health Canada at 1-866-234-2345.

Read the entire patient information overview for Pfizerpen (Penicillin G potassium)

What is Prescribing information?

The FDA package insert formatted in easy-to-find categories for health professionals and clinicians.

Pfizerpen FDA Prescribing Information: Side Effects
(Adverse Reactions)


Penicillin is a substance of low toxicity but does have a significant index of sensitization. The following hypersensitivity reactions have been reported: skin rashes ranging from maculopapular eruptions to exfoliative dermatitis; urticaria; and reactions resembling serum sickness, including chills, fever, edema, arthralgia and prostration. Severe and occasionally fatal anaphylaxis has occurred (see WARNINGS).

Hemolytic anemia, leucopenia, thrombocytopenia, nephropathy, and neuropathy are rarely observed adverse reactions and are usually associated with high intravenous dosage. Patients given continuous intravenous therapy with penicillin G potassium in high dosage (10 million to 100 million units daily) may suffer severe or even fatal potassium poisoning, particularly if renal insufficiency is present. Hyperreflexia, convulsions, and coma may be indicative of this syndrome.

Cardiac arrhythmias and cardiac arrest may also occur. (High dosage of penicillin G sodium may result in congestive heart failure due to high sodium intake.)

The Jarisch-Herxheimer reaction has been reported in patients treated for syphilis.

Read the entire FDA prescribing information for Pfizerpen (Penicillin G potassium)

Report Problems to the Food and Drug Administration


You are encouraged to report negative side effects of prescription drugs to the FDA. Visit the FDA MedWatch website or call 1-800-FDA-1088.

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