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Ampicillin vs. amoxicillin: Differences, similarities, and which is better for you

Drug overview & main differences | Conditions treated | Efficacy | Insurance coverage and cost comparison | Side effects | Drug interactions | Warnings | FAQ

If you have ever had a bacterial infection, chances are you have taken an antibiotic. Ampicillin and amoxicillin are antibiotics used in the treatment of various bacterial infections. Both medications are approved by the U.S. Food and Drug Administration (FDA). Antibiotics have antimicrobial activity and are used in the treatment of bacterial infections only—they are not effective in treating viral infections (like the flu or common cold).

Ampicillin and amoxicillin are classified in a group of medications called penicillin (or aminopenicillin), or beta-lactam antibiotics. They work by preventing bacteria from forming cell walls, leading to the death of the bacteria. Both antibiotics are used to treat a variety of bacterial infections (more on this below).

Although ampicillin and amoxicillin are both penicillin antibiotics, they are not exactly the same. Continue reading to learn more about ampicillin and amoxicillin.

What are the main differences between ampicillin and amoxicillin?

Ampicillin is a penicillin antibiotic. It is used to treat many gram-positive and gram-negative bacteria. The brand name of ampicillin is Principen; however, Principen is no longer available as a brand-name drug. The medication is available as the generic, ampicillin, as an oral capsule or as an injection. Ampicillin is also available in injection form as Unasyn, which contains ampicillin along with sulbactam (to prevent antibiotic resistance). Unasyn is no longer available as the brand—it is available as generic ampicillin/sulbactam only.

Amoxicillin is also a penicillin antibiotic. It is chemically similar to ampicillin and has a broad spectrum of activity against many gram-positive and gram-negative bacteria.

The brand name of amoxicillin is Amoxil; however, Amoxil is no longer commercially available. The medication is only available in the generic form of amoxicillin. Amoxicillin is most commonly prescribed as amoxicillin capsules for adults, or suspension for children, or in combination with clavulanic acid (which prevents antibiotic resistance) as a medication called Augmentin.

It’s important to remember that when you take an antibiotic, you should take it as directed, and finish the full course, even if you are feeling better before the treatment is over. However, if you have been taking your antibiotic for several days and you are not feeling any better or you are feeling worse, contact your healthcare provider for guidance.

Main differences between ampicillin and amoxicillin
Ampicillin Amoxicillin
Drug class Penicillin (beta-lactam) antibiotic Penicillin (beta-lactam) antibiotic
Brand/generic status Generic Generic
What is the brand name? Principen (no longer available in brand name) Amoxil, Trimox (no longer available in brand name)
What form(s) does the drug come in? Ampicillin: capsule, injection

Unasyn: (ampicillin-sulbactam): injection

Amoxicillin: capsule, suspension, tablet, chewable tablet

Augmentin: (amoxicillin-clavulanate): tablet, chewable tablet, suspension

Prevpac: a course of treatment containing amoxicillin capsules in combination with lansoprazole and clarithromycin (used for stomach ulcers caused by H. pylori bacteria)

What is the standard dosage? Example: ampicillin 500 mg every 6 hours for 10-14 days Example: amoxicillin 500 mg 3 times a day for 10 days
How long is the typical treatment? 10-14 days; may vary 7-10 days; may vary
Who typically uses the medication? Adults and children Adults and children

Conditions treated by ampicillin and amoxicillin

Ampicillin is used to treat bacterial infections including:

  • Genitourinary tract infections, including gonorrhea, caused by coli, P. mirabilis, enterococci, Shigella, S. typhosa and other Salmonella and non penicillinase-producing N. gonorrhoeae
  • Respiratory tract infections caused by non penicillinase-producing influenzae and staphylococci, and streptococci including Streptococcus pneumoniae
  • Gastrointestinal tract infections caused by Shigella, S. typhosa and other Salmonella, E. coli, P. mirabilis, and enterococci
  • Meningitis caused by Meningitidis

Amoxicillin is used to treat a variety of bacterial infections:

  • Ear infections (otitis media), nose infections, or throat infections caused by certain strains of Streptococcus, pneumoniae, Staphylococcus spp., or Haemophilus influenzae
  • Urinary tract infections caused by Escherichia coli, P. mirabilis, or Enterococcus faecalis
  • Skin infections or skin structure infections caused by certain strains of Streptococcus, Staphylococcus (such as Staphylococcus aureus), or E. coli
  • Lower respiratory tract infections caused by certain strains of Streptococcus, Streptococcus pneumoniae, Staphylococcus, or H. influenzae
  • Acute uncomplicated gonorrhea in males and females caused by Neisseria gonorrhoeae
  • Eradication of pylori to reduce the risk of duodenal ulcer recurrence
  • Amoxicillin is also used as triple therapy with lansoprazole and clarithromycin (as Prevpac) in patients with pylori infection and duodenal ulcer

Antibiotics such as ampicillin or amoxicillin should only be used in bacterial infections when determined to be appropriate by your healthcare provider. This helps prevent antibiotic resistance. The Centers for Disease Control and Prevention (CDC) is promoting the appropriate use of antibiotics by helping healthcare providers choose the appropriate antibiotic and reduce unnecessary antibiotic use when treating various infectious diseases. This is called antibiotic stewardship.

Is ampicillin or amoxicillin more effective?

Studies comparing the two drugs are not recent and/or used a very small sample size. One study, from 1974, compared the two drugs for ear infection in children and found both drugs to be similarly effective. Amoxicillin was better tolerated, with fewer side effects than ampicillin.

Ampicillin alone is not prescribed as much as it was in the past, due to the development of drug resistance. Currently, amoxicillin is much more widely prescribed. However, ampicillin in combination with sulbactam (Unasyn) is commonly given as an injection in the hospital setting. Sulbactam is a drug called a beta-lactamase inhibitor and helps prevent drug resistance. In the outpatient setting, this is similar to a patient taking Augmentin, which is amoxicillin plus clavulanic acid, a beta-lactamase inhibitor.

The most effective medication can be determined by your healthcare provider, who can diagnose your infection as bacterial or viral. If the infection is bacterial, the decision on which antibiotic to use will be based on what bacteria is causing the infection (if known, or if not known, what bacteria is suspected to be causing the infection). Your prescriber will look at your full medical history as well as other medical conditions you have and other medications you take that may interact with ampicillin or amoxicillin.

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Coverage and cost comparison of ampicillin vs. amoxicillin

Ampicillin and amoxicillin are covered by most insurance plans and Medicare Part D.

A typical ampicillin prescription would be for 40, 500 mg capsules. The out-of-pocket price would be about $30. Using a SingleCare card for ampicillin can lower the price to less than $20.

A typical prescription of amoxicillin would be for 30, 500 mg capsules. The out-of-pocket price can be more than $20. With an amoxicillin SingleCare discount coupon you may pay as little as $5.

  Ampicillin Amoxicillin
Typically covered by insurance? Yes Yes
Typically covered by Medicare Part D? Yes Yes
Standard dosage 40, 500 mg capsules 30, 500 mg capsules
Typical Medicare Part D copay $0-$1 $0-$1
SingleCare cost $20+ $5+

Common side effects of ampicillin vs. amoxicillin

The most common side effects of ampicillin and amoxicillin are related to penicillin sensitivity and are more likely to occur in patients who have previously had a hypersensitivity to penicillins and in patients with a history of allergy and/or asthma. The most common side effects include nausea, vomiting, diarrhea, and skin rash/hypersensitivity reactions. Serious anaphylactic reactions require emergency treatment.

In general, antibiotic treatment may make you more susceptible to diarrhea or a yeast infection. Ask your healthcare provider if you should take a probiotic.

This is not a full list of side effects—other side effects may occur. Consult your healthcare provider for a full list of adverse events.

  Ampicillin Amoxicillin
Side effect Applicable? Frequency Applicable? Frequency
Diarrhea Yes Not reported Yes >1%
Nausea Yes Not reported Yes >1%
Abdominal pain Yes Not reported Yes Not reported
Vomiting Yes Not reported Yes >1%
Rash Yes Not reported Yes >1%

Source: DailyMed (ampicillin), DailyMed (amoxicillin), FDA label (amoxicillin)

Drug interactions of ampicillin vs. amoxicillin

Ampicillin and amoxicillin have a similar list of drug interactions because they are structurally similar medications.

Taking ampicillin or amoxicillin with an anticoagulant such as warfarin may affect bleeding—patients should be monitored if on this combination. Also, allopurinol, a gout medication, in combination with ampicillin or amoxicillin can increase the chance of developing a rash.

Oral contraceptives (also known as birth control pills) may be less effective when taken in combination with antibiotics such as ampicillin or amoxicillin. Consult your healthcare provider about the need for backup birth control (such as a condom) while you are taking an antibiotic.

Ampicillin and amoxicillin both interact with the live oral typhoid vaccine, Vivotif Berna. The antibiotic may inactivate the vaccine.

This is not a complete list of drug interactions—other drug interactions may occur. Consult your healthcare provider for a full list of drug interactions.

Drug Drug class Ampicillin Amoxicillin
Warfarin Anticoagulant Yes Yes
Allopurinol Xanthine oxidase inhibitor (used for gout) Yes Yes
Oral contraceptives Oral contraceptives Yes Yes
Probenecid Uricosuric Yes Yes
Vivotif Berna Typhoid vaccine (live) Yes Yes
Bupropion Aminoketone antidepressant Yes Yes
Methotrexate Antimetabolite Yes Yes
Magnesium citrate Saline laxative Yes Yes

Warnings of ampicillin and amoxicillin

Warnings for ampicillin and amoxicillin:

  • Clostridium difficile-associated diarrhea has been reported with most antibiotics and can range in severity from mild diarrhea to fatal colitis. This diarrhea may occur during or after antibiotic use (even several months later). If you experience diarrhea, nausea, abdominal pain, and/or fever, contact your healthcare provider immediately.
  • If you have a history of an allergic reaction to penicillins, do not take ampicillin or amoxicillin.
  • Serious, occasionally fatal, hypersensitivity reactions (anaphylaxis) have been reported with penicillins. This may also occur in patients who are being treated with a cephalosporin such as cephalexin. Patients should not be prescribed ampicillin or amoxicillin if there has been a previous reaction. If an allergic reaction occurs, ampicillin or amoxicillin should be discontinued immediately and you should seek emergency treatment.
  • Ampicillin or amoxicillin should only be used to treat a bacterial infection. Using an antibiotic for a viral infection will not treat the illness, and can also lead to antibiotic resistance.
  • Prolonged use of antibiotics may lead to fungal infection, which will need to be treated.

Additional warnings of ampicillin:

  • Patients with both gonorrhea and syphilis also need appropriate parenteral penicillin (penicillin G) treatment.
  • Despite treatment with ampicillin, the patient may still need a surgical procedure, particularly in staphylococcal infections.

Frequently asked questions about ampicillin vs. amoxicillin

What is ampicillin?

Ampicillin is a beta-lactam, penicillin antibiotic, used to treat many different bacterial infections in adults and children. Unasyn contains ampicillin and sulbactam. It is available in injection form only. Sulbactam is a beta-lactamase inhibitor, which is added to ampicillin in Unasyn to prevent antibiotic resistance.

What is amoxicillin?

Amoxicillin is a beta-lactam antibiotic, related to penicillin, used to treat a variety of bacterial infections in adults and children. Amoxicillin is a very common prescription antibiotic. Augmentin (which contains amoxicillin and clavulanic acid) is another very common antibiotic prescribed for many bacterial infections. Clavulanic acid is a beta-lactamase inhibitor, which is added to amoxicillin in Augmentin to help prevent antibiotic resistance.

Are ampicillin and amoxicillin the same?

Ampicillin and amoxicillin are very similar. They are structurally similar to each other and are in the same drug class. They have similar side effects and drug interactions, but some different indications and different dosing. You can read more about the two drugs in the information outlined above.

Is ampicillin or amoxicillin better?

Both drugs can be effective when used on their own; however, ampicillin is more susceptible to drug resistance, so amoxicillin is more widely prescribed. However, that does not mean it is “better,” on its own, but more likely to be successful against the bacteria it is intended to attack. When either drug is used in combination with a beta-lactamase inhibitor (Unasyn as an injection or Augmentin as an oral medication), the antimicrobial coverage is more effective, and the risk of drug resistance is lower.

Can I use ampicillin or amoxicillin while pregnant?

Both medications are pregnancy category B. There are no adequate and well-controlled studies in pregnant women. Your healthcare provider will weigh risks vs benefits when deciding if you need an antibiotic, and which antibiotic is right for you when you are pregnant.

Can I use ampicillin or amoxicillin with alcohol?

While ampicillin and amoxicillin are not contraindicated with alcohol, note that alcohol can prevent your body from fighting an infection. Alcohol can also make gastrointestinal side effects worse. It’s best to avoid alcohol until you are feeling better.

Is ampicillin a strong antibiotic?

Ampicillin can be effective against a variety of bacteria that cause certain infections. However, it is not prescribed as much as it used to be, because of the issue of resistance when used alone as ampicillin. It is often used (as an injection) in the hospital setting as Unasyn, which contains sulbactam in addition to ampicillin, to help prevent bacterial resistance.

How many days should ampicillin be taken?

The number of days depends on the type and severity of the infection and will be determined by your healthcare provider. Usually, ampicillin will be taken for about 10 to 14 days. Be sure to finish the full course of antibiotics—do not stop abruptly even if you are feeling better because the bacteria could return.

What bacteria does ampicillin kill?

Ampicillin is used to treat various bacterial infections including certain urinary tract infections, respiratory tract infections, gastrointestinal tract infections, and meningitis. See the above section “Conditions treated by ampicillin and amoxicillin” for more information on the types of bacteria that ampicillin works against in each of these types of infections.