Ampicillin — Compare Prices at US & Canada Pharmacies

Ampicillin and amoxicillin are both members of the penicillin class of antibiotics, more specifically aminopenicillins within the broader beta-lactam family — both work by disrupting bacterial cell wall formation, which kills the bacteria. They're related, but not interchangeable: the two differ in how well they're absorbed, how they're given, and what they're typically used for.

Ampicillin was originally sold under the brand name Principen, no longer available commercially — today it's generic-only, as oral capsules or injectable formulations. Combined with sulbactam (a beta-lactamase inhibitor that helps overcome bacterial resistance), it's marketed as Unasyn, also available generically as ampicillin/sulbactam.

Amoxicillin was originally branded as Amoxil, also discontinued — it's generic-only too, typically as capsules for adults or liquid suspension for children, or — combined with clavulanic acid — as Augmentin.

Both drugs were discovered in the 1950s, so neither is meaningfully "newer" than the other. Amoxicillin's dominant use today comes down to better gut absorption, not a more recent discovery — ampicillin remains the hospital standard specifically because an injectable form is readily available and its particular spectrum (including Enterococcus and Listeria) is still needed there.

According to its FDA-approved labeling, ampicillin is indicated for bacterial infections including:

• Respiratory tract infections caused by non-penicillinase-producing strains of Haemophilus influenzae, Staphylococcus species, and Streptococcus (including Streptococcus pneumoniae)
• Urinary tract infections caused by Escherichia coli and Proteus mirabilis
• Gastrointestinal infections caused by Shigella, Salmonella typhosa, other Salmonella species, E. coli, P. mirabilis, and enterococci
• Meningitis caused by Neisseria meningitidis

Amoxicillin's labeling covers a similar but distinct list:

• Ear, nose, and throat infections caused by susceptible strains of Streptococcus, S. pneumoniae, Staphylococcus species, or Haemophilus influenzae
• Urinary tract infections caused by E. coli, P. mirabilis, or Enterococcus faecalis
• Skin and soft tissue infections caused by susceptible Streptococcus, Staphylococcus species (including Staphylococcus aureus), or E. coli
• Lower respiratory tract infections caused by Streptococcus, S. pneumoniae, Staphylococcus species, or H. influenzae
• Helicobacter pylori eradication (combined with lansoprazole and clarithromycin)

Both are strictly for confirmed or suspected bacterial infections — using either for a viral illness won't help and contributes to antibiotic resistance.

In most outpatient care, amoxicillin is preferred because it reaches higher, more stable blood levels when taken by mouth than oral ampicillin does — making it the better fit for common infections like ear infections, sinusitis, strep throat, community-acquired pneumonia, and dental infections. Ampicillin, by contrast, is poorly absorbed from the gut and stays in the intestines longer, which is actually a therapeutic advantage for intestinal infections caused by Salmonella and Shigella.

Ampicillin also has an injectable form that amoxicillin doesn't, so it's the one used in hospitals — often paired with sulbactam to counter bacterial resistance (Unasyn), the same idea behind amoxicillin's outpatient pairing with clavulanic acid (Augmentin).

Which one is "more effective" isn't really the right question — it depends on confirming the infection is bacterial in the first place, then identifying the specific organism, ideally through culture and sensitivity testing (of urine, blood, or sputum), or clinical guidelines and local resistance patterns when testing isn't practical. Your provider will also weigh your medical history and other medications before choosing.

Both are typically covered by commercial insurance and Medicare Part D. Without insurance, the cash price for a 28-capsule bottle of 500 mg ampicillin runs around $14.51, versus roughly $11 or more for a 21-capsule bottle of 500 mg amoxicillin — see the price tables above for current per-pharmacy figures for ampicillin specifically. Discount programs and comparing prices across pharmacies can lower the out-of-pocket cost for either one.

The most frequent side effects of both drugs involve the gastrointestinal tract and allergic reactions, particularly in anyone with a history of penicillin sensitivity — nausea, vomiting, diarrhea, and skin rash or other signs of hypersensitivity are typical. Severe anaphylactic reactions are uncommon but require immediate emergency care.

Both can also disrupt your normal bacterial balance, raising the risk of diarrhea or a secondary yeast infection — ask your provider whether a probiotic makes sense for you during treatment. This isn't a complete list; ask your provider or pharmacist for the full picture of possible side effects.

Because the two share a similar chemical structure, their interaction profiles are similar too. Combined with anticoagulants like warfarin, either drug may alter bleeding risk and calls for monitoring. Allopurinol (for gout) raises the likelihood of a rash when combined with either. Current evidence suggests neither drug meaningfully reduces how well oral contraceptives work, though severe vomiting or diarrhea — from the infection or as a side effect — can interfere with pill absorption, so a backup method is worth discussing if you develop GI symptoms. Both also interact with the live oral typhoid vaccine (Vivotif Berna), which either antibiotic can inactivate. This isn't an exhaustive list — ask your provider or pharmacist about your specific medications.

A personal or family history of allergic reaction to penicillins rules out both drugs — serious and occasionally fatal hypersensitivity reactions, including anaphylaxis, have occurred with penicillin-class drugs, and cross-reactivity with cephalosporins (like cephalexin) is possible in penicillin-allergic patients. Stop the medication and seek emergency care right away if you notice signs of an allergic reaction.

C. difficile–associated diarrhea has been reported with most antibiotics, ranging from mild to life-threatening, and can start during treatment or emerge months later — report diarrhea, nausea, abdominal cramping, or fever to your provider promptly. Prolonged use of either drug can also lead to fungal overgrowth requiring separate treatment.

Neither drug has an absolute contraindication with alcohol, but drinking can suppress immune function and worsen GI side effects like nausea and diarrhea, so it's best to limit or avoid alcohol until you've recovered. Both are routinely prescribed in pregnancy — well-controlled safety studies are limited for either, so your provider will weigh the infection being treated against any risks before prescribing.

Sources:

• dailymed.nlm.nih.gov
• dailymed.nlm.nih.gov
• www.cdc.gov
• www.cdc.gov
• www.idsociety.org
• www.cdc.gov

Yes. Anyone with a documented allergic reaction to either ampicillin or amoxicillin should be considered allergic to both, and in practice is typically labeled with a penicillin-class allergy — meaning the whole penicillin family should be avoided, not just the specific drug that caused the reaction.