Key takeaways
Ampicillin and amoxicillin are antibiotics in the penicillin family, specifically aminopenicillins, with a broad spectrum of activity against various bacteria.
The most common side effects of ampicillin and amoxicillin are nausea, vomiting, diarrhea, and skin rashes. Serious reactions require emergency treatment.
Ampicillin and amoxicillin are covered by most insurance plans and Medicare Part D, but amoxicillin is prescribed more often because it is better absorbed when taken orally.
Ampicillin and amoxicillin are classified as penicillins (subcategory aminopenicillins), which are within the broader group of beta-lactam antibiotics. They work by preventing bacteria from forming cell walls, leading to their death.
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 the same. Continue reading to learn more about ampicillin and amoxicillin.
Ampicillin vs. amoxicillin at a glance |
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|---|---|
| Ampicillin | Amoxicillin |
| Less commonly prescribed | Most commonly prescribed aminopenicillin |
| Usually every 6 hours | Usually every 8-12 hours |
| Often IV in hospitals | Mostly oral |
| Lower oral absorption | Better oral absorption |
| Used for Enterococcus/Listeria | Used for ear infections, sinusitis, strep throat, pneumonia |
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 former brand name of ampicillin was Principen, but Principen is no longer available as a brand-name drug. The medication is available as a generic oral capsule or as an injection. Ampicillin is a component of the intravenous antibiotic Unasyn, which contains ampicillin along with sulbactam (to circumvent antibiotic resistance). Unasyn is also available as generic ampicillin/sulbactam.
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 was 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 capsules for adults, suspension for children, or, in combination with clavulanic acid (which circumvents 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 complete the full course, even if you feel better before the treatment is over. However, if you have been taking your antibiotic for several days and are not feeling any better, or are feeling worse, contact your healthcare provider for guidance.
Main differences between ampicillin vs. amoxicillin |
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|---|---|---|
| Category | 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 (no longer available in brand name): 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 adult dosage? | Example: ampicillin 500 mg every 6 hours for 7 days | Example: amoxicillin 500 mg 3 times a day for 7 days |
| How long is the typical treatment? | 7 days; may vary | 7 days; may vary |
| Who typically uses the medication? | Adults and children | Adults and children |
Conditions treated by ampicillin and amoxicillin
According to the drug label, ampicillin is used to treat bacterial infections, including:
- Respiratory tract infections caused by non-penicillinase-producing H. influenzae and staphylococci, and streptococci, including Streptococcus pneumoniae
- Urinary tract infections caused by E. coli and P. mirabilis
- Gastrointestinal tract infections caused by Shigella, S. typhosa, and other Salmonella, E. coli, P. mirabilis, and enterococci
- Meningitis caused by N. Meningitidis
Amoxicillin’s drug label cites that it is approved by the Food and Drug Administration (FDA) to treat a variety of bacterial infections:
- Ear infections (otitis media), nose infections, or throat infections caused by certain strains of Streptococcus, S. pneumoniae, Staphylococcus spp., or Haemophilus influenzae
- Urinary tract infections (UTIs) caused by Escherichia coli, P. mirabilis, or Enterococcus faecalis
- Skin infections or skin structure infections caused by certain strains of Streptococcus, Staphylococcus spp. (such as Staphylococcus aureus), or E. coli
- Lower respiratory tract infections caused by certain strains of Streptococcus, Streptococcus pneumoniae, Staphylococcus spp., or H. influenzae
- Eradication of H. pylori to reduce the risk of duodenal ulcer recurrence when used with lansoprazole and clarithromycin
Antibiotics such as ampicillin or amoxicillin should be used only for bacterial infections when determined 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.
Which antibiotic works better?
In most outpatient infections, amoxicillin is preferred because it is absorbed better after oral administration and achieves higher, more consistent blood concentrations than oral ampicillin. Common examples of oral amoxicillin use include ear infections, sinus infections, strep throat, pneumonia, and dental infections. Because oral ampicillin is not well absorbed from the intestinal tract and therefore remains in the gut longer, it is used to treat intestinal infections caused by the bacteria Salmonella and Shigella.
Unlike amoxicillin, which has no injectable form, ampicillin is commonly administered by injection, often in combination with sulbactam in hospital settings. Sulbactam is a beta-lactamase inhibitor that helps kill bacteria resistant to ampicillin alone. In the outpatient setting, this is similar to a patient taking Augmentin, which is amoxicillin plus clavulanic acid, another beta-lactamase inhibitor.
The most effective medication can be determined by your healthcare provider, who can first diagnose your infection as bacterial or viral. Antibiotics are ineffective against viruses and can provoke side effects and resistance if overused. If the infection is bacterial, the decision about which antibiotic to use will be based on which bacteria are causing it. This can be determined by taking a culture from the site of infection, such as a urine sample, and then determining antibiotic sensitivities.
More commonly, a healthcare provider chooses an antibiotic based on national guidelines, such as those from the Infectious Disease Society of America (IDSA), or local patterns of antibiotic resistance among bacteria. Before making a decision, your prescriber will also review your full medical history, including any other medical conditions you have and any medications you take that may interact with ampicillin or amoxicillin.
Is ampicillin or amoxicillin cheaper?
Most insurance plans and Medicare prescription plans cover ampicillin and amoxicillin.
The average price for ampicillin without insurance would be about $22 for 28, 500 mg capsules. The average price for amoxicillin without insurance can be more than $11 for 21, 500 mg capsules.
You can save up to 80% on these antibiotics and other prescriptions with a free SingleCare coupon.
Cost and coverage: Ampicillin vs. amoxicillin |
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|---|---|---|
| Insurance & cost details | Ampicillin | Amoxicillin |
| Typically covered by insurance? | Yes | Yes |
| Typically covered by Medicare Part D? | Yes | Yes |
| Standard dosage | 28, 500 mg capsules | 21, 500 mg capsules |
| Typical Medicare Part D copay | $0-$1 | $0-$1 |
| SingleCare coupons | See latest prices | See latest prices |
Common side effects of ampicillin vs. amoxicillin
The most common side effects of ampicillin and amoxicillin are gastrointestinal disturbances, as well as penicillin sensitivity, which is more likely to occur in patients with a prior hypersensitivity to penicillins. 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.
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, when combined with ampicillin or amoxicillin, can increase the risk of developing a rash.
Most evidence indicates that ampicillin and amoxicillin do not significantly reduce the effectiveness of hormonal birth control. However, vomiting or severe diarrhea caused by illness or antibiotics could reduce the absorption of oral contraceptives. Ask your healthcare provider if backup contraception should be used.
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.
Warnings of 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, healthcare providers avoid prescribing ampicillin or amoxicillin.
- Serious, occasionally fatal, hypersensitivity reactions (anaphylaxis) have been reported with penicillins. Penicillin-allergic patients may also react to cephalosporin antibiotics, such as cephalexin. If an allergic reaction occurs, discontinue ampicillin or amoxicillin immediately and seek emergency treatment.
- Ampicillin or amoxicillin should be used only to treat bacterial infections. 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.
Frequently asked questions about ampicillin vs. amoxicillin
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 they have different oral absorption, indications, and dosing. You can read more about the two drugs in the information outlined above.
Can you be allergic to one if you’re allergic to the other?
Anyone who has had an allergic reaction to ampicillin or amoxicillin is considered to be allergic to both. In fact, they are typically labeled as allergic to the entire penicillin class of antibiotics.
Do ampicillin and amoxicillin treat the same infections?
Other than in hospital settings, most people are prescribed oral antibiotics, and since amoxicillin is much better absorbed from the intestinal tract than ampicillin, it is chosen for most infections susceptible to aminopenicillins. Examples include ear, sinus, and lung infections. Intestinal infections caused by Shigella or Salmonella are notable exceptions and are treated with oral ampicillin.
Is amoxicillin just a newer version of ampicillin?
Both amoxicillin and ampicillin were discovered in the 1950s. Amoxicillin, being more commonly used, is simply a result of better absorption into the bloodstream after oral dosing. Conversely, in hospital settings, ampicillin products are more commonly used due to the availability of intravenous formulations.
Is ampicillin or amoxicillin better?
Both drugs can be effective when used on their own; however, ampicillin is not absorbed as well orally, so amoxicillin is more widely prescribed. 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 commonly used in pregnancy. The drugs’ labels do note that there are no adequate and well-controlled studies in pregnant women. Your healthcare provider will weigh the risks and benefits when deciding whether you need an antibiotic and which antibiotic is right for you while you are pregnant.
Can I use ampicillin or amoxicillin with alcohol?
While ampicillin and amoxicillin are not contraindicated with alcohol, drinking alcohol may weaken your body’s ability to fight an infection. Alcohol can also make gastrointestinal side effects worse. It’s best to avoid alcohol until you are feeling better.
Are ampicillin and amoxicillin strong antibiotics?
Ampicillin and amoxicillin are broad-spectrum antibiotics, but that does not make them “strong”. “Strong” is not a medical term used to describe antibiotics. Your healthcare provider will prescribe an appropriate antibiotic based on the bacteria involved.
How many days should ampicillin or amoxicillin 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 and amoxicillin are taken for 7 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 and amoxicillin kill?
Ampicillin and amoxicillin are used to treat various bacterial infections, including certain urinary tract, respiratory tract, and gastrointestinal tract infections, as well as meningitis. See the above section “Conditions treated by ampicillin and amoxicillin” for more information on the bacterial species that these antibiotics work against in each of these types of infections.
The SingleCare prices in this article are the most accurate at the time of publishing in ZIP code 23666 as of Sep. 10, 2026. Prices vary by pharmacy. Visit our coupon page for updated drug prices at pharmacies near you.
- Ampicillin drug label, NIH DailyMed (2025)
- Amoxicillin drug label, NIH DailyMed (2024)
- Antibiotic use and stewardship in the United States, 2025 update: Progress and opportunities, CDC (2026)
- Healthy habits: Antibiotic do’s and don’ts, CDC (2025)
- Practice guidelines, IDSA (2026)
- About C. diff, CDC (2026)
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