Head-to-head clinical analysis & difference comparison: details on mechanism of action, dosing, half-life, interactions, and maternal-fetal safety.
Ampicillin vs AMOXIL
Clinician-reviewed, head-to-head comparison of mechanism, dosing, pharmacokinetics, and safety profiles.
Last clinically reviewed: July 2026 · OpiCalc Medical Review Team
Ampicillin is a penicillin antibiotic that inhibits bacterial cell wall synthesis by binding to penicillin-binding proteins (PBPs), inhibiting transpeptidase activity, and disrupting peptidoglycan cross-linking.
Amoxicillin inhibits bacterial cell wall synthesis by binding to penicillin-binding proteins (PBPs), inhibiting transpeptidation and activating autolytic enzymes, leading to bacterial lysis.
Respiratory tract infections,Urinary tract infections,Meningitis,Septicemia,Endocarditis,Gastrointestinal infections,Intra-abdominal infections,Skin and soft tissue infections,Prophylaxis for bacterial endocarditis (off-label),Listeriosis
Upper respiratory tract infections (e.g., otitis media, sinusitis, pharyngitis/tonsillitis) due to susceptible streptococci, pneumococci, and H. influenzae,Lower respiratory tract infections (e.g., pneumonia, bronchitis) due to susceptible streptococci, pneumococci, and H. influenzae,Genitourinary tract infections (e.g., uncomplicated gonorrhea, cystitis) due to susceptible E. coli, P. mirabilis, and enterococci,Skin and skin structure infections due to susceptible streptococci, staphylococci, and E. coli,Helicobacter pylori eradication (as part of combination therapy),Lyme disease (early localized or early disseminated),Prophylaxis of infective endocarditis (dental procedures) in patients with certain cardiac conditions
250-500 mg orally every 6 hours; 1-2 g IV/IM every 4-6 hours.
250-500 mg orally every 8 hours or 500-875 mg orally every 12 hours; for severe infections, up to 500 mg every 8 hours or 875 mg every 12 hours.
Terminal elimination half-life: 1-1.8 hours in adults with normal renal function; prolonged to 7-20 hours in end-stage renal disease (Cr Cl <10 m L/min).
Terminal half-life: 1-1.5 hours (normal renal function); prolonged to 7-20 hours in anuria; neonates: 3-4 hours.
Ampicillin is primarily excreted unchanged in the urine via renal tubular secretion and glomerular filtration. A small portion is metabolized by hydrolysis to penicilloic acid, but hepatic metabolism is minimal.
Amoxicillin is primarily metabolized through hydrolysis of the beta-lactam ring to inactive penicilloic acid, accounting for 60-70% of the dose; about 10% is metabolized via hepatic pathways to amoxicilloic acid; renal excretion as unchanged drug is 60-80% via tubular secretion and glomerular filtration.
Renal: 90% unchanged via glomerular filtration and tubular secretion; biliary: 10% (small amount).
Renal: 60-80% unchanged via tubular secretion and glomerular filtration; Biliary/fecal: minor, <5% excreted in bile; dose adjustment in Cr Cl <30 m L/min.
17-20% bound to serum albumin.
17-20%, primarily to albumin.
0.28-0.31 L/kg (higher in neonates and critically ill patients).
0.3-0.4 L/kg; indicates distribution into total body water.
Oral: 50% (fasting); reduced by 25-50% with food. IM: ~100% (complete absorption).
Oral: 75-90% (variable with food, decreased absorption); IM: near 100%.
Cr Cl 10-50 m L/min: administer every 6-12 hours; Cr Cl <10 m L/min: administer every 12-24 hours.
GFR 10-30 m L/min: 250-500 mg every 12 hours; GFR <10 m L/min: 250-500 mg every 24 hours; hemodialysis: 250-500 mg every 24 hours with an additional dose after dialysis.
No adjustment needed for hepatic impairment; dose as in normal hepatic function.
No dose adjustment required for mild to moderate hepatic impairment (Child-Pugh class A or B); caution in severe hepatic impairment (Child-Pugh class C) due to limited data.
Neonates: 25-50 mg/kg IV/IM every 12 hours (first week), every 8 hours (1-4 weeks); Infants/Children: 25-100 mg/kg/day IV/IM divided every 6-8 hours; Oral: 50-100 mg/kg/day divided every 6-8 hours.
Neonates ≤28 days: 25-30 mg/kg/day divided every 12 hours; Infants and children >28 days: 20-40 mg/kg/day divided every 8 hours; for otitis media: 50-90 mg/kg/day divided every 8-12 hours.
Cr Cl >50 m L/min: no adjustment; Cr Cl 10-50 m L/min: administer every 6-12 hours; Cr Cl <10 m L/min: administer every 12-24 hours; maximum 2 g/day.
No specific dose adjustment based solely on age; monitor renal function and adjust dose based on creatinine clearance (Cr Cl) as per renal adjustment guidelines; maintain adequate hydration.
Serious and occasionally fatal hypersensitivity reactions (anaphylaxis) have been reported in patients on penicillin therapy. Clostridium difficile-associated diarrhea (CDAD) has been reported with use of nearly all antibacterial agents.
None
Serious hypersensitivity reactions (anaphylaxis) requiring emergency treatment,Clostridium difficile-associated diarrhea (CDAD) may occur,Prolonged use may result in superinfection with non-susceptible organisms,Use with caution in patients with renal impairment (dose adjustment needed),Skin rash is common in patients with mononucleosis or concurrent allopurinol use
Serious hypersensitivity reactions (anaphylaxis) can occur; contraindicated in patients with penicillin allergy,Clostridium difficile-associated diarrhea (CDAD) may occur, ranging from mild diarrhea to fatal colitis,Prolonged use may result in superinfection with resistant organisms,Hepatic dysfunction and cholestatic jaundice (rare),Skin rashes, including morbilliform rash (common in patients with mononucleosis),Decreased efficacy when used with bacteriostatic agents (e.g., tetracyclines, chloramphenicol),Use with caution in patients with renal impairment (Cr Cl <30 m L/min) due to increased risk of seizures with high doses
Hypersensitivity to ampicillin or any penicillin,Hypersensitivity to cephalosporins (cross-allergenicity possible),Infections caused by penicillinase-producing bacteria (including methicillin-resistant Staphylococci)
Known hypersensitivity to amoxicillin, penicillins, or any component of the formulation,Mononucleosis (high incidence of morbilliform rash)
Food decreases absorption of oral ampicillin; take on an empty stomach. No specific food restrictions aside from timing. Avoid alcohol as it may increase gastrointestinal irritation.
No significant food interactions; absorption is not altered by food. Avoid excessive alcohol as it may increase risk of GI side effects and hepatotoxicity (rare).
Ampicillin is FDA Pregnancy Category B. Animal studies have not demonstrated teratogenic effects. In humans, ample data across all trimesters indicate no increased risk of major birth defects, though there is a theoretical risk of altered gut flora and maternal-fetal effects from high doses. No documented teratogenicity from controlled studies in pregnant women.
Penicillins, including amoxicillin, are generally considered low risk in pregnancy. Animal studies have not shown teratogenic effects. In humans, data from large cohort studies and meta-analyses do not indicate an increased risk of major congenital malformations, preterm birth, or low birth weight. Use is acceptable if clinically indicated across all trimesters.
Ampicillin is excreted in breast milk in low concentrations. The milk-to-plasma (M/P) ratio is approximately 0.2–0.3. Amount ingested by the infant is estimated to be <0.1% of a therapeutic neonatal dose. The American Academy of Pediatrics considers it compatible with breastfeeding. Monitor infant for potential diarrhea, rash, or candidiasis.
Amoxicillin is excreted into breast milk in small amounts, with an M/P ratio of approximately 0.02-0.05. The estimated dose to the infant is less than 1% of the maternal weight-adjusted dose. It is generally considered compatible with breastfeeding. However, potential risks include infant sensitization, diarrhea, and rash. Monitor for these effects.
Pregnancy increases renal clearance and volume of distribution for ampicillin, potentially lowering serum concentrations. For standard infections, no dose adjustment is routinely needed. However, for serious infections (e.g., meningitis, endocarditis), higher doses or more frequent intervals may be required to achieve therapeutic levels. Consider therapeutic drug monitoring in severe cases.
Physiologic changes in pregnancy (e.g., increased renal blood flow, GFR, and volume of distribution) may reduce serum concentrations of amoxicillin. While standard dosing may be effective, some experts recommend using the higher end of the dosing range or more frequent dosing for severe infections. However, no specific dose adjustment is routinely required; clinical response should guide therapy.
Ampicillin is a bactericidal antibiotic that inhibits cell wall synthesis. It is effective against Gram-positive cocci (except penicillinase-producing staphylococci) and some Gram-negative bacilli. Use with probenecid to increase serum levels. Monitor for rash, which may indicate mononucleosis. Dose adjustment required in renal impairment (Cr Cl <30 m L/min). Administer IV slowly over 10-15 minutes to avoid phlebitis.
Amoxicillin is a first-line agent for acute otitis media, streptococcal pharyngitis, and uncomplicated community-acquired pneumonia. It has a time-dependent killing mechanism; optimal efficacy requires maintaining serum concentrations above the MIC for >40% of the dosing interval. Dose adjustment is necessary for creatinine clearance <30 m L/min. It is compatible with clavulanate for beta-lactamase coverage. Rash during therapy may indicate non-allergic ampicillin rash (especially with viral infections) or true hypersensitivity; assess carefully.
Take ampicillin exactly as prescribed, even if you feel better.,Complete the full course of therapy to prevent resistance.,Take on an empty stomach (1 hour before or 2 hours after meals) for best absorption.,Oral suspension must be refrigerated; shake well before each use.,Discard any unused oral suspension after 14 days.,Report any skin rash, diarrhea, or difficulty breathing to your doctor immediately.,Do not use if you are allergic to penicillins or cephalosporins.,Avoid alcohol while on this medication to reduce risk of side effects.,May reduce the effectiveness of oral contraceptives; use additional birth control.
Take exactly as prescribed; complete the full course even if you feel better.,May be taken with or without food; avoid large meals if GI upset occurs.,Report any rash, especially if accompanied by hives or difficulty breathing.,Do not use leftover antibiotics; discard after completing course.,Use additional contraception if on oral contraceptives (may reduce efficacy).,For suspension: shake well, measure dose with provided device, refrigerate and discard after 14 days.
"The coadministration of ampicillin, a broad-spectrum penicillin antibiotic, with streptozocin, a nitrosourea antineoplastic agent used in pancreatic islet cell carcinoma, may reduce serum concentrations of streptozocin. This interaction is hypothesized to result from ampicillin-induced alterations in gut microbiota, leading to reduced enterohepatic recirculation of streptozocin metabolites or interference with renal tubular secretion of the active drug. Clinically, this could diminish the anticancer efficacy of streptozocin, potentially compromising tumor response."
"Ampicillin may reduce the serum concentration of Kanamycin via direct chemical inactivation in body fluids, particularly in patients with renal impairment. This interaction can lead to subtherapeutic aminoglycoside levels, potentially compromising antibacterial efficacy and promoting bacterial resistance. Clinically, this necessitates careful monitoring of Kanamycin levels and dose adjustments to maintain therapeutic effect."
"Ampicillin, a beta-lactam antibiotic, can reduce the serum concentration of plicamycin, an antineoplastic antibiotic, when co-administered. This interaction likely occurs due to ampicillin-induced alterations in gut microbiota, which may decrease the enterohepatic recirculation of plicamycin, leading to reduced systemic exposure. The resulting subtherapeutic plicamycin levels may compromise its antitumor efficacy and increase the risk of treatment failure."
No interactions on record
Explore head-to-head clinical comparisons of other medications in the same therapeutic classes.
Common clinical questions about Ampicillin vs AMOXIL, answered by our medical review team.
Ampicillin is a Penicillin Antibiotic that works by Ampicillin is a penicillin antibiotic that inhibits bacterial cell wall synthesis by binding to penicillin-binding proteins (PBPs), inhibiting transpeptidase activity, and disrupting peptidoglycan cross-linking.. AMOXIL is a Penicillin Antibiotic that works by Amoxicillin inhibits bacterial cell wall synthesis by binding to penicillin-binding proteins (PBPs), inhibiting transpeptidation and activating autolytic enzymes, leading to bacterial lysis.. They differ in pharmacokinetic profiles, FDA-approved indications, and side effect profiles.
Potency comparisons between Ampicillin and AMOXIL depend on the specific clinical indication. These are both Penicillin Antibiotic agents and are not directly interchangeable by dose. A physician or clinical pharmacist should guide any therapeutic switching decisions.
The standard adult dose of Ampicillin is: 250-500 mg orally every 6 hours; 1-2 g IV/IM every 4-6 hours.. The standard adult dose of AMOXIL is: 250-500 mg orally every 8 hours or 500-875 mg orally every 12 hours; for severe infections, up to 500 mg every 8 hours or 875 mg every 12 hours.. Dosing should always be individualized based on indication, renal and hepatic function, age, and other patient factors.
No direct drug-drug interaction has been formally documented between Ampicillin and AMOXIL in current clinical databases. However, individual patient risk factors including other medications, organ function, and comorbidities should always be evaluated by a qualified healthcare provider.
The maternal-fetal safety profiles differ. Ampicillin is classified as Category A/B. Ampicillin is FDA Pregnancy Category B. Animal studies have not demonstrated teratogenic effects. In humans, ample data across all trimesters indicate no increased risk of major bi. AMOXIL is classified as Category C. Penicillins, including amoxicillin, are generally considered low risk in pregnancy. Animal studies have not shown teratogenic effects. In humans, data from large cohort studies and. Always consult a maternal-fetal medicine specialist before taking either drug during pregnancy or lactation.