All You Need to Know About Penicillin

By Terez Malka, MD
Medically reviewed checkmarkMedically reviewed
January 7, 2022

Penicillin is one of the most commonly used antibiotics in the world.

Discovered by chance nearly 100 years ago in a London laboratory, it’s been used clinically for over 75 years and is still widely prescribed to treat certain types of bacterial infections, including pneumonia, STDs, and more.

Though it is an effective treatment option for many infections, there are certain risks and side effects possible when taking this medicine.

To ensure safety of use, it’s important to follow the instructions given by your health care professional. 

And while penicillin does require a prescription from a licensed provider, that doesn’t mean that it requires an in-person visit.

Knowing when and how to connect with a health care provider online can help you to get a prescription for penicillin when needed.

What is Penicillin?

Penicillin is a narrow-spectrum antibiotic used to treat bacterial infections.

Specifically, penicillin is effective against gram-positive bacteria, including:

  • Streptococcus pneumoniae
  • Groups A, B, C, and G streptococci
  • Nonenterococcal group D streptococci
  • Virdians group streptococci
  • Non-penicillinase producing staphylococcus

It can be used to treat a variety of infections caused by bacteria, including:

Like other antibiotics, penicillin will not treat a cold, flu, COVID-19, or other viral infections.

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How Does Penicillin Work?

Put simply, penicillin works by killing bacteria.

More specifically, it works by inhibiting the synthesis of the bacterial peptidoglycan cell wall.

Most bacteria have a peptidoglycan cell wall that surrounds the plasma membrane of bacterial cells and helps to provide structural integrity.

As bacterial cells multiply and grow, this wall is continually remodeled. 

But when penicillin inhibits the synthesis of this cell wall, bacteria are weakened as osmotic pressure pushes water into the cell, eventually killing it.

What Are the Types of Penicillin?

There are several types of penicillins, including natural, extended-spectrum, penicillinase-stable, and aminopenicillins.


Natural penicillins were the first antibiotics to be used in clinical practice and are based on the original penicillin-G structure.

Natural penicillins are effective against gram positive and gram negative bacteria, including:

  • Staphylococci
  • Streptococci
  • Meningococci
  • Treponema
  • Borrelia
  • Leptospira


Extended-spectrum penicillins are semisynthetic penicillins derived from ampicillin.

They have a wider spectra of activity than natural penicillins and are more effective against gram negative organisms, including those that cause burn-related infections.

However, they are less active against gram positive and anaerobic organisms than penicillin G. 


Also referred to as second-generation penicillins, penicillinase-resistant penicillins are stable to the bacterial enzyme beta-lactamase, making them the penicillins of choice for most infections caused by staphylococci and streptococci bacteria.


Aminopenicillins have an additional amino group that enhances its antibacterial activity.

In fact, aminopenicillins were the first penicillins to have activity against some gram-negative organisms, including:

  • H. influenzae
  • Escherichia coli
  • Proteus mirabilis
  • Salmonella
  • Shigella

How to Take Penicillin

Before taking penicillin, it’s important to follow the directions on the prescription label carefully.

If any of the directions are unclear, ask your provider or pharmacist about how to proceed.

Penicillin V is taken as a tablet or as an oral solution, while penicillin G is often administered intravenously or intramuscularly by a health care provider. 

When taking penicillin V at home, it’s important to store the medicine at room temperature and away from excess heat and moisture.

The exact dosage and timing of penicillin medications will vary depending on what condition you are treating.

For infections, penicillin is generally taken every 6 hours (four times a day) or every 8 hours (three times a day) until the course is completed.

For the treatment of rheumatic fever, penicillin is generally taken twice a day. 

In most cases, it’s a good idea to take penicillin at about the same time each day.

If you miss a dose, be sure to take it as soon as you remember, unless it’s almost time for the next one. In that case, skip the missed dose and continue the regular dosing schedule.

Never take a double dose to make up for a missed one.

Side Effects of Penicillin

Penicillin can cause some mild side effects, including:

If any of these symptoms gets worse or won’t go away, reach out to a healthcare professional.

Importantly, experiencing these side effects doesn’t necessarily mean that you have an allergy to penicillin.

If you do have side effects, always let your health care provider know when you’re being prescribed antibiotics in the future. 

Risks of Penicillin

There are some risks of penicillin use.

Before taking penicillin, tell your provider about your complete medical and medication history, including any chronic health conditions you may have, such as kidney disease, asthma, hay fever, or other allergies.

Allergic Reaction

Allergic reaction is a reasonable concern when taking penicillin, which is why it’s important to disclose whether or not you’re allergic to penicillin or any other antibiotics.

If you do have an allergy, it’s helpful if you know what your reaction was, because some symptoms, such as nausea, are not a true allergy.

According to the Centers for Disease Control and Prevention (CDC), approximately 10% of all patients in the United States report having an allergic reaction to penicillin in the past, but in reality, most patients do not have true IgE-mediated reactions.

In fact, experts estimate that fewer than 1% of the population are truly allergic to penicillins.

Signs of a true IgE-mediated (type 1) allergic reaction to penicillin include:

  • Reactions that occur immediately or within one hour of use
  • Multiple pink or red hives that are intensely itchy
  • Angioedema: Localized edema without hives that affect the stomach, face, extremities, genitalia, oropharynx, or larynx
  • Wheezing
  • Shortness of breath
  • Anaphylaxis (which requires changes or symptoms in at least two of the following systems):
    • Skin: Hives, flushing, itching, and/or angioedema
    • Respiratory: Cough, nasal congestion, shortness of breath, chest tightness, wheeze, sensation of throat closure or choking, change in voice quality
    • Cardiovascular: Hypotension, faintness, tachycardia, tunnel vision, chest pain, sense of doom, loss of consciousness
    • Gastrointestinal: Nausea, vomiting, abdominal cramping, diarrhea

If you’re unsure whether or not you have a true penicillin allergy, ask your provider about penicillin skin testing and an oral challenge dose.


Overdosing is possible when taking penicillin, which is why it’s important to take the medicine exactly as directed. 

Symptoms of a penicillin overdose include:

  • Agitation
  • Confusion
  • Jerking movements
  • Seeing things or hearing voices
  • Seizures
  • Coma
  • Weakness
  • Fast, slow, or irregular heartbeat

If you experience any signs of a penicillin overdose, call the poison control helpline at 1-800-222-1222 or reach out to your health care provider immediately.

If you suspect a loved one or friend is unresponsive due to a penicillin overdose, call emergency services at 911.

Drug Interactions

It’s important to tell your provider about any medications, therapies, vitamins, supplements, or herbal products you’re currently taking and whether or not you’re pregnant or breastfeeding before starting medication. 

Four major interactions with penicillin V include the following therapies and vaccines:

  • Bacillus Calmette-Guerin (Bcg), used to treat early-stage bladder cancer
  • Cholera vaccine, live
  • Methotrexate, a chemotherapy and immunosuppressive drug
  • Typhoid vaccine, live

Antibiotic Resistance

Antibiotic resistance is a serious and important concern for providers and patients worldwide.

Today, antibiotic-resistant bacteria infect more than two million people in the United States, often resulting in hospitalizations or, in some cases, death. 

To fight antibiotic resistance, the CDC designated better-informed use and prescription of antibiotics as a national priority.

You can do your part by taking antibiotics like penicillin exactly as prescribed, not sharing your antibiotics with anyone or saving them, and never pressuring a health care provider to prescribe antibiotics.

Where to Get Penicillin

Purchasing penicillin requires a prescription from a licensed health care provider, which you can obtain in-person or online.

Before obtaining a prescription, your provider will likely want to discuss your symptoms, medical history, and family history, either during an in-person visit or during a telehealth visit over phone or video.

Though less likely, sometimes your provider may suggest running specific tests to determine which antibiotic is right for you.

Thankfully, getting a prescription for penicillin online, when appropriate, is simple.

With K Health, simply download the app to check your symptoms, explore conditions and treatments, and if needed, text with a doctor in minutes.

Other online sources may work similarly.

Just be sure that you are connected with a board-certified medical professional.

Antibiotics online

Our physicians can prescribe antibiotics for various conditions, but only if necessary. Chat with a provider now.

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When to See a Doctor

If you’re experiencing unexplained symptoms and want to screen for and/or rule out a bacterial infection, reach out to your provider.

If you’ve already started antibacterial medication such as penicillin, reach out to your provider immediately if you experience any of the possible severe side effects, including:

  • Rash
  • Hives
  • Itching
  • Hoarseness
  • Wheezing
  • Difficulty swallowing or breathing
  • Joint pain
  • Swelling of the throat, tongue or lips
  • A return of any of the symptoms of your original infection (including fever, sore throat, chills, etc.)
  • Severe diarrhea

Additionally, if you don’t experience an improvement in symptoms after five days of penicillin treatment, contact your provider for more information. 

How K Health Can Help 

Did you know you can get affordable primary care with the K Health app? Download K Health to check your symptoms, explore conditions and treatments, and if needed text with a provider in minutes. K Health’s AI-powered app is based on 20 years of clinical data.

Frequently Asked Questions

What is the most common use for penicillin?
Penicillin is used to treat a variety of bacterial infections, including meningitis, pneumonia, gonorrhea, syphilis, and endocarditis.
Can penicillin be used as a painkiller?
No, penicillin is not used as a painkiller. Penicillin is used to treat bacterial infections. It use requires a prescription from a licensed health care provider and should not be used as a painkiller. However, if being used to treat a bacterial infection that causes pain, effectively treating the infection should improve your symptoms, including alleviating pain.
What is considered the strongest antibiotic for bacterial infection?
The strongest and most effective antibiotic will be the one prescribed by your provider to treat your specific bacterial infection. Different types of antibiotics (including different types of penicillins) are designed to treat specific infections, which is why it’s important to follow your provider’s instructions when treating a bacterial infection.

K Health articles are all written and reviewed by MDs, PhDs, NPs, or PharmDs and are for informational purposes only. This information does not constitute and should not be relied on for professional medical advice. Always talk to your doctor about the risks and benefits of any treatment.

Terez Malka, MD

Dr. Terez Malka is a board-certified pediatrician and emergency medicine physician.

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