Penicillin is a class of antibiotic medications. Penicillins treat bacterial infections like strep throat, ear infections and urinary tract infections. They work by attaching to and damaging the cell walls of bacteria. Types of penicillin include penicillin V, penicillin G, amoxicillin and ampicillin.


What is penicillin?

Penicillin is a group of antibiotics (medications used to treat bacterial infections) that work in a specific way to destroy bacteria in your body. Types of penicillin and drugs closely related to them are called “penicillins.” They’re a subclass of beta-lactam antibiotics.

Penicillin antibiotics come in oral form (pills or liquid you swallow) and IV form (liquid that a provider injects directly into your vein).

What is penicillin used for?

Providers use penicillin to treat certain bacterial infections, including:


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What are the types of penicillin?

Two of the major differences between the types of penicillin are the way they’re made and what kind of bacteria they’re effective against. Types of penicillin include:

  • Natural penicillins. Natural penicillins are found in the world around us (they aren’t made by people). Scientists isolate (purify) just the penicillin from where it’s found to make it into medications.
  • Semi-synthetic penicillins. Scientists alter penicillin’s natural form to make more effective antibiotics. These are called semi-synthetic penicillins. They include penicillinase-resistant penicillins, aminopenicillins and extended-spectrum penicillins.
  • Combination medications. Penicillins are often combined with other medications into one drug to help them work better.

Natural penicillins

Penicillin G and penicillin V (also known as penicillin V potassium) are natural penicillins. Providers use natural penicillins to treat a range of infections, including strep throat, syphilis and Lyme disease. Penicillin G comes in IV form. Penicillin V you can take by mouth.

Semi-synthetic penicillins

Semi-synthetic penicillins include:

  • Penicillinase-resistant penicillins. Nafcillin, oxacillin and dicloxacillin are penicillinase-resistant penicillins. They come in both IV and pill form and are often used to treat staph infections. Methicillin, which is rarely used anymore, is also a penicillinase-resistant penicillin.
  • Aminopenicillins. Amoxicillin and ampicillin are aminopenicillins. Amoxicillin is one of the most commonly used penicillins. Providers use it to treat ear infections, UTIs, pneumonia and other common infections. It comes in a pill or liquid you can swallow. Ampicillin comes in pill or IV form.
  • Extended-spectrum penicillins. Piperacillin is an extended-spectrum penicillin. Providers use it for hard-to-treat infections, like Pseudomonas aeruginosa. Other extended-spectrum penicillins, like carbenicillin and ticarcillin, are discontinued in the U.S.

Combination penicillins

Penicillin can be combined with other drugs, called beta-lactamase inhibitors, to help them work better. Beta-lactamase inhibitors work by preventing bacterial enzymes (beta-lactamases) from destroying the antibiotic. Some penicillin/beta-lactamase inhibitor combinations include:

  • Augmentin® (amoxicillin and clavulanic acid).
  • Unasyn® (ampicillin and sulbactam).
  • Zosyn® (piperacillin and tazobactam).

How does penicillin work?

Penicillin works by attaching to the wall of bacteria cells. It damages the cell wall and eventually destroys the bacteria.

Over time, bacteria have developed resistance to natural penicillins. This means that certain types of bacteria can prevent antibiotics from damaging their cells. This has happened many times since penicillin’s first use. Each time, scientists developed new antibiotics that worked in the same ways as natural penicillin — by attaching to the cell wall — but had methods for avoiding antibiotic resistance.

Some bacteria, like MRSA (methicillin-resistant Staphylococcus aureus), resist all types of penicillins. This can make them very hard to treat. Scientists have developed antibiotics that work in different ways to try to treat these infections.

How long will I need penicillin?

Providers usually prescribe penicillin medications for one to three weeks, depending on which infection you have. It’s important to take penicillins as prescribed. Take them on schedule until you finish the entire course, even if you feel better.


Risks / Benefits

What are the side effects of penicillin?

The most common side effects of penicillins include:

  • Nausea.
  • Vomiting.
  • Diarrhea.
  • Skin rash.
  • Changes in taste or smell.
  • Headache.

What are the risks of penicillin?

The most common risks of taking penicillins include:

Penicillin allergy

The most serious risk of penicillin medications is an allergic reaction. Experts estimate that only about 1% of people have a penicillin allergy, even though about 10% of people report being allergic. If you experience vomiting or trouble breathing, call 911 or get emergency medical attention right away — you might be having an allergic reaction.

C. diff

Some people can develop the bacterial infection Clostridioides difficile (C. diff) from taking antibiotics. While it sounds like the opposite of what should happen, penicillin antibiotics don’t kill off C. diff but they can kill off bacteria that prevent C. diff from growing out of control. C. diff infections cause severe diarrhea, stomach pain and fever and can be hard to treat. Go to the emergency room if you have severe diarrhea and stomach pain while taking antibiotics.

Antibiotic resistance

Bacteria are constantly developing ways to protect themselves from antibiotics (antibiotic resistance). This is dangerous because it makes bacterial infections harder to treat, and more likely to cause life-threatening complications.

Taking penicillins and other antibiotics when you don’t need to or not taking them properly can increase the risk that bacteria will develop resistance. If you stop taking penicillin before you’ve completed the course, you might not kill all the bacteria. The remaining bacteria might develop resistance, making your infection harder to treat.


Recovery and Outlook

How long will it take for me to feel better?

Ask your provider what to expect and how long it might take to feel better. If you have a common bacterial infection (like strep throat or an ear infection), you might feel better in just a couple of days. Other infections take longer to recover from. Make sure you take your entire course of medicine, even if you feel better.

Is there anything I can do to make taking penicillin easier on me?

Taking penicillins with food can help reduce side effects like nausea and vomiting. You may also be able to take over-the-counter anti-nausea or anti-diarrheal medications to manage side effects. Ask your provider before you take any additional medications.

When To Call the Doctor

When should I call my healthcare provider?

Contact your provider if you:

  • Have questions about how to take your medication.
  • Are having side effects that are making it difficult to take your medication (like nausea or stomach pain).
  • Have severe diarrhea. Go to the emergency room if you have watery diarrhea multiple times per day or signs of dehydration.
  • Develop a rash or other signs of an allergic reaction. Call 911 or seek immediate medical attention if you’re vomiting or having trouble breathing.

Additional Common Questions

Who discovered penicillin?

Alexander Fleming first discovered penicillin in 1928, when he left Petri dishes growing bacteria while he was away. When he returned, he noticed there weren’t any bacteria near where mold was growing on the dish. The mold had something in it that was killing bacteria.

It was very difficult to purify the bacteria-killing substance from the mold and produce it into a drug. Though it was successful at treating bacterial infections in trials, penicillin wasn’t available for public use until 1946.

Since Fleming’s discovery, scientists have isolated two types of natural penicillin and developed many semi-synthetic penicillins.

What’s the difference between amoxicillin and penicillin?

Amoxicillin is a form of penicillin. It’s the most commonly prescribed antibiotic. Scientists developed it to treat types of bacterial infections that other penicillins hadn’t been able to in the past.

A note from Cleveland Clinic

Fleming’s discovery of penicillin changed the outlook of bacterial infections from life-threatening to treatable. Most people will take a type of penicillin for an ear infection, strep throat, UTI or other common infection at some point in their lives. But antibiotic resistance can lead to dangerous, hard-to-treat infections. Make sure to take medications as prescribed and contact your provider if you have any concerns.

Medically Reviewed

Last reviewed on 03/14/2024.

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