Differences Between Amoxicillin and Penicillin

By Terez Malka, MD
Medically reviewed checkmarkMedically reviewed
January 24, 2022

Penicillins are a class of antibiotics used against a wide range of bacteria. Both penicillin and amoxicillin are penicillin-class drugs.

While the medicines penicillin V and penicillin G are naturally occurring penicillins, amoxicillin was made by chemically modifying penicillins to make them more powerful.

As a result, amoxicillin tends to treat a broader range of bacterial infections. 

For the sake of this article, we will only refer to the natural penicillins (G and V) as penicillin.

Your doctor may prescribe amoxicillin or penicillin depending on your symptoms and the infection being treated.  

It’s important to know the major differences between amoxicillin and penicillin, their effectiveness, and precautions to follow while on the medications.

What Are Antibiotics?

Antibiotics are drugs used to treat bacterial infections.

They work by either destroying the bacterial cells or inhibiting their growth.

Antibiotics do not treat viral infections like the flu, colds, or COVID-19.  

There are different types of antibiotics, each with their own spectrum of activity.

Here are some types of antibiotics and how they work: 

  • Fluoroquinolones: They work by inhibiting the actions of enzymes in the bacteria responsible for DNA synthesis. Examples include ciprofloxacin and levofloxacin. 
  • Macrolides: They stop bacteria growth by inhibiting protein synthesis. Examples include erythromycin and azithromycin. 
  • Aminoglycosides: They kill bacteria by inhibiting protein synthesis. Examples include gentamicin and streptomycin. 
  • Tetracyclines: Bacteria need protein to grow. Tetracyclines stop bacterial growth by inhibiting bacterial protein synthesis. Examples include doxycycline and minocycline. 
  • Cephalosporins: Bacteria cell walls keep their structure intact. Cephalosporins destroy bacteria by inhibiting enzymes necessary in their cell wall formation. Examples include cefixime and cefuroxime. 

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Amoxicillin is a penicillin-type antibiotic. It is a broad spectrum bactericidal antibiotic, meaning it works against a wide range of bacterial infections by killing the bacteria that cause them. 

It’s a prescription-only medicine, which means you cannot legally purchase it over the counter, without a doctor’s prescription.

Popular amoxicillin brands include Amoxil, Moxilin, and Amoxicot.

This antibiotic can be combined with potassium clavulanate (another active ingredient) in the form of amoxicillin-potassium clavulanate for better efficacy.


Penicillin G and V are natural penicillins and the first penicillins ever used.

They are prescription-only medications used to treat a wide range of bacterial infections. 

Penicillin G is available as penicillin G benzathine, procaine, sodium, and potassium and used as an intravenous or intramuscular injection to treat severe bacterial infections.

It is sold under brand names like Bicillin L-A and Bicillin C-R.

Penicillin V potassium (phenoxymethyl penicillin can be taken orally to treat common bacterial infections.

Popular penicillin V brands include Penicillin VK and Veetids. 

What Conditions Do They Treat? 

Amoxicillin and penicillins treat the same bacterial infections with a few exceptions.

The table below shows examples of conditions treated by amoxicillin and penicillin. 

ConditionsAmoxicillin Penicillin V
Peptic ulcer disease x
Scarlet feverxx
Bacterial respiratory infectionsxx
Skin infections x, when combined with clavulanic acid
Dental infectionsxx
Urinary tract infectionsx, when combined with clavulanic acid

Mild to moderate respiratory infections include streptococcal pharyngitis (strep throat), bacterial sinusitis (most is viral and does not require antibiotics), and community acquired pneumonia

From this table, we see that while amoxicillin and penicillin treat similar infections, there are slight differences in their effectiveness for certain conditions. 

What Are Their Forms and Dosage?

Amoxicillin and penicillin come in different forms and dosages.

Your doctor or health care provider will determine the ideal drug form and dosage for you. 

Here are the general forms and commonly used dosages of amoxicillin and penicillin. 


Amoxicillin can be taken orally as capsules or tablets.

Chewable tablets or oral suspensions (liquid) also exist for those unable to swallow tablets or capsules. 

Amoxicillin is available in the following forms and strengths:

  • Oral tablets: Available as 500mg and 875mg tablets.
  • Oral capsules: Available as 250mg and 500mg capsules.
  • Powder for oral suspension: Available as 125mg/5 mL, 200mg/5 mL, 250mg/5 mL, and 400mg/5 mL.
  • Oral chewable tablets: Available 125 mg and 250 mg tablets.

Amoxicillin dosage depends on the condition being treated.

Your doctor or provider may prescribe a higher dose of amoxicillin for certain types of infections, such as ear infections.

Only use amoxicillin (or any antibiotic) according to your prescribed dosage.

Here are some common dosages for amoxicillin.

These may vary based on specific circumstances: 

  • Adult dose: Adults are typically prescribed 750-1750mg/day in divided doses every 8-12 hours.
  • Children: Pediatric amoxicillin prescriptions are usually based on body weight, and typically range from 40 to 90mg/kg/day divided every 8-12 hours. Your child’s health care provider may prescribe chewable tablets or oral suspensions of varying doses depending on your child’s age and body weight.
  • Dosage for H.pylori infection (stomach ulcer): For stomach ulcers, amoxicillin is prescribed as part of a triple therapy (i.e. with two other medications). The dosage is 1g amoxicillin, 30 mg lansoprazole, and 500 mg clarithromycin, to be taken twice daily (every 12 hours) for 14 days.


Penicillin V potassium can be taken orally and exists in the following forms:

  • Oral tablets: Available as 250mg and 500mg tablets. 
  • Oral solution: Available as 125mg/5mL and 250mg/5mL solutions.

The typical adult dose for oral penicillin v tablets is 250-500mg, taken every 6 to 8 hours for 7 to 20 days. 

Penicillin G benzathine, procaine, potassium and sodium are available for intravenous or intramuscular use.

The dosage of intravenous penicillin G is 300,000 to 4 million units every 6 to 8 hours.

Benzathine penicillin G is prescribed for the treatment of syphilis as a one time dose of 2.4 million units.

Your doctor or health care provider will administer this dose.

What Are Costs and Coverage? 

Amoxicillin and penicillin are relatively inexpensive drugs.

If you think you might get a prescription for either of these medications and are curious about the cost, this section discusses all you need to know about their cost and coverage.


The cost of amoxicillin depends on your treatment plan, the pharmacy you visit and your insurance coverage.

On average, 500mg of amoxicillin with 4 capsules costs $10, and a 20-tablet pack costs around $13. 

Talk to your doctor or pharmacist if you’re unsure of how much amoxicillin will cost you.

They’ll be able to determine the cost of each pill you’re getting. 

Most Medicare and insurance plans cover the cost of generic amoxicillin. 


Penicillin is covered by medicare and health insurance plans.

Penicillin V potassium oral tablets (500mg) cost around $29 for a 6-tablet pack, depending on the pharmacy you visit. 

Penicillin G potassium injectable powder for injection costs around $50 per unit.

The price will vary depending on the pharmacy you visit and whether or not you have coupons. 

What Are Common Side Effects? 

Amoxicillin and penicillin both have some side effects to watch out for.

Some of these side effects are shown in the table below. 

Side effectsAmoxicillin Penicillin Rare or common (R or C) 
Vomiting xxC
Mild skin rashxxC
Diarrhea xxC
Stomach upsetxxC
Vaginal candidiasisxxC
Difficulty swallowing xxR
Itching xxC
Difficulty breathing xxR
Nose bleeds xxR

Speak to your health care provider if you experience side effects while taking either of these drugs.

They’ll let you know whether to continue with the medication or switch to something else.

What Are Possible Drug Interactions? 

Drug interactions are changes in the efficacy of a medicine when it is taken with certain other drugs.

Your medicines can become more or less effective when taken with other medications or even certain foods.

Some drugs may interact with amoxicillin and penicillin during therapeutic use.

The table below shows a few drugs that interact with amoxicillin and penicillin. 

DrugsDrug ClassAmoxicillin Penicillin 
Birth control pills(levonorgestrel) Oral contraceptives/Birth control pillsxx
Allopurinol,ProbenecidGout medicationx
AmilorideDiuretic xx
Methotrexate Antimetabolitesxx
Azithromycin, erythromycin, clarithromycinAntibioticsxx

Before beginning a dose of antibiotics, let your health care provider know if you’re on any other medications.

Which is More Effective? 

A study compared the effectiveness of amoxicillin and penicillin in the treatment of streptococcal tonsillopharyngitis.

Another study compared oral amoxicillin to injectable penicillin in the treatment of children with severe pneumonia. 

Both results showed that amoxicillin and penicillin have equivalent effects when used to treat the same bacterial infection.

However, amoxicillin and penicillin each have certain conditions  for which they are most effective.

Your health care provider can determine which is the correct antibiotic for your condition.


Amoxicillin is effective against a variety of gram-positive bacteria and some gram-negative bacteria. 

Here are some major conditions treated using amoxicillin:

  • Ear, nose and throat infections: Amoxicillin is effective in the treatment of streptococcal pharyngitis, tonsillitis, and otitis media in adults and children. 
  • Stomach ulcers: Amoxicillin is effective in treating stomach ulcers by destroying the causative bacteria Helicobacter pylori. Amoxicillin is used here in a triple therapy with clarithromycin and lansoprazole, or in a dual therapy with lansoprazole only. This treatment destroys Helicobacter pylori and reduces the risk of recurrence. 

Amoxicillin is also effective in the treatment of:

  • Bacterial lower respiratory tract infections, such as community acquired pneumonia
  • Acute bacterial sinusitis
  • Skin and skin structure infections, when combined with clavulanic acid
  • Urinary tract infections, when combined with clavulanic acid
  • Lyme disease 


Penicillin V potassium is used to treat bacterial respiratory tract infections, bacterial throat infections, gum, and mouth infections.  

Penicillin V potassium is also effective against rheumatic fever.

Rheumatic fever is a serious health issue that may develop after a scarlet fever or strep throat infection.

It causes swelling of the heart valves and also affects the joints, skin, and brain.

Penicillin V potassium can prevent rheumatic fever from reoccurring. 

Penicillin G is prescribed in the treatment of pneumonia, staph infections, strep throat, meningitis, gonorrhea and syphilis

Penicillin G is also used to treat the following infections:

  • Septicemia and sepsis: This is a condition where bacteria enters the bloodstream. 
  • Pericarditis: This is an infection of the tissues around the heart called the pericardium. 
  • Endocarditis: This can be caused by infection of the inner lining of the heart chambers or valves. 
  • Haverhill fever: This is an infection caused by bites or scratches from rodents.
  • Pasteurella infection: This is a skin and soft tissue infection caused by animal bites or scratches. 

Precautions and Warnings

Amoxicillin and penicillin are generally safe to take.

However there are some precautions you should be aware of while taking either of these antibiotics.  


Some factors to be considered when taking this medicine include:

  • Allergies: Talk to your health care provider If you notice any allergic reactions while on this medication. Some allergic reactions to amoxicillin include hives and difficulty breathing. 
  • Breastfeeding: Taking amoxicillin while breastfeeding is unlikely to harm your baby,  although occasional cases of diarrhea and thrush have been reported in infants. Weigh the risks and benefits carefully before taking this medication while breastfeeding. 
  • Pediatric: Amoxicillin is safe to take in children, but caution should be exercised in children 3 months old and younger. Infants this age do not have well-developed kidney functions and amoxicillin is excreted through urine. 
  • Drug interactions: It is important to let your doctor know the other medications you’re on before they prescribe amoxicillin. Taking some medications together with amoxicillin can lead to potentially dangerous drug interactions. Examples of drugs you shouldn’t take with amoxicillin are warfarin, doxycycline, sulfasalazine, and tetracycline. 

Amoxicillin also makes oral birth control ineffective, so you should find an alternative means of birth control while you’re taking it. 


Here are some precautions to keep in mind while taking penicillin:

  • Allergies: Stop taking this drug if you notice any allergic reactions to it. You should also use this drug with caution if you have asthma, have had any hypersensitivity reactions in the past, or if you’ve had Steven Johnson syndrome. Severe allergic reactions to penicillin can lead to anaphylaxis. 

If you’re unsure whether or not you’re allergic to penicillin, you can take a skin allergy test to see if you’re allergic to penicillin if you are concerned that you might have an allergy. 

  • Kidney disease: Penicillin is excreted through the kidneys, so if you have any kidney diseases, seek medical advice from your doctor. You may need a lower dose of penicillin to reduce the strain on your kidneys. If you’re on hemodialysis, your doctor may prescribe a higher dose of penicillin because the rates of excretion of penicillin will be higher on hemodialysis. 

Antibiotics Resistance 

Antibiotic resistance is a public health issue and a cause for concern. It happens when organisms like bacteria and fungi are no longer affected by the antibiotics meant to kill them.

It’s the bacteria, not the body, that develops this resistance. 

With antibiotic resistance, antibiotics become ineffective against bacteria, which makes the infection harder or impossible to treat.

Antibiotic-resistant infections usually require a longer treatment period and higher antibiotic doses to treat. 

Antibiotic resistant bacteria can also multiply and make other bacteria resistant as well

Taking your antibiotics incorrectly, taking antibiotics when you do not need them (ie for viral illnesses), skipping doses, or abandoning your medication halfway can all make it more likely for antibiotic resistance to develop.

Additionally, antibiotic resistance can spread fast among people and get to new locations and countries — making a whole range of antibiotics useless. 

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When to See a Doctor

If you think you or someone else has taken too much amoxicillin or penicillin, watch out for overdose symptoms like severe diarrhea and vomiting, and contact your health care provider or poison control center  for medical advice.

You should also seek medical advice if you notice an allergic reaction to these medicines.

Some common allergic reactions to antibiotics include hives, difficulty breathing, or difficulty swallowing. 

Remember to see your doctor if you’re about to start on another medication that may interact with your antibiotics. 

How K Health Can Help 

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Frequently Asked Questions

Can you take amoxicillin if you're allergic to penicillin?
No, you cannot take amoxicillin if you're allergic to penicillin because the drugs belong to the same class.
What is the strongest type of antibiotic?
There is no single most powerful antibiotic. The strength of an antibiotic depends on the infection you’re treating.
Is it possible to outgrow an allergy to penicillin?
While true allergies to penicillins are likely to be lifelong, most people who think they have penicillin allergies because of a childhood reaction do not have true allergy to penicillins. Speak with your healthcare provider if you think you may have had a penicillin allergy and are wondering if you can safely take these antibiotics- they can arrange for allergy testing or a safe trial of the medication.

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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