Head-to-head clinical analysis & difference comparison: details on mechanism of action, dosing, half-life, interactions, and maternal-fetal safety.
AMOXICILLIN vs Ampicillin
Clinician-reviewed, head-to-head comparison of mechanism, dosing, pharmacokinetics, and safety profiles.
Last clinically reviewed: July 2026 · OpiCalc Medical Review Team
Amoxicillin is a beta-lactam antibiotic that inhibits bacterial cell wall synthesis by binding to penicillin-binding proteins (PBPs), inhibiting transpeptidase activity, and activating autolytic enzymes.
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.
Upper respiratory tract infections (e.g., otitis media, sinusitis, pharyngitis/tonsillitis),Lower respiratory tract infections (e.g., community-acquired pneumonia, acute exacerbation of chronic bronchitis),Genitourinary tract infections (e.g., cystitis, urethritis),Skin and skin structure infections,Helicobacter pylori eradication (in combination with clarithromycin and a proton pump inhibitor),Lyme disease (early localized),Prophylaxis of infective endocarditis (for dental procedures in high-risk patients),Off-label: Anthrax (post-exposure prophylaxis), uncomplicated gonorrhea
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
250-500 mg orally every 8 hours or 500-875 mg orally every 12 hours; for severe infections, up to 1 g orally every 8 hours.
250-500 mg orally every 6 hours; 1-2 g IV/IM every 4-6 hours.
Terminal elimination half-life: 1-1.5 hours in normal renal function. Prolonged to 7-20 hours in end-stage renal disease.
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).
Amoxicillin is primarily metabolized by hydrolysis to penicilloic acid (inactive). It is not extensively metabolized by the liver; about 60% of an oral dose is excreted unchanged in urine.
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.
Renal: 60-80% unchanged via glomerular filtration and tubular secretion. Biliary: up to 20% excreted in bile. Fecal: minimal.
Renal: 90% unchanged via glomerular filtration and tubular secretion; biliary: 10% (small amount).
17-20% bound to serum albumin.
17-20% bound to serum albumin.
0.3-0.4 L/kg. Distributes well into most body fluids and tissues, including pleural, peritoneal, and synovial fluids; limited CNS penetration unless meninges inflamed.
0.28-0.31 L/kg (higher in neonates and critically ill patients).
Oral: 74-92% (absorption is not food-dependent). IM: approximately 100%.
Oral: 50% (fasting); reduced by 25-50% with food. IM: ~100% (complete absorption).
Cr Cl 30-50 m L/min: 250-500 mg every 8-12 hours. Cr Cl 10-29 m L/min: 250-500 mg every 12 hours. Cr Cl <10 m L/min: 250-500 mg every 24 hours. Hemodialysis: 250-500 mg every 24 hours, supplemented during and after dialysis.
Cr Cl 10-50 m L/min: administer every 6-12 hours; Cr Cl <10 m L/min: administer every 12-24 hours.
No dose adjustment required for mild to moderate hepatic impairment. Severe hepatic impairment (Child-Pugh class C): use with caution; specific dosing guidelines not established.
No adjustment needed for hepatic impairment; dose as in normal hepatic function.
Children >3 months: 20-40 mg/kg/day divided every 8 hours for mild to moderate infections; 40-90 mg/kg/day divided every 8-12 hours for severe infections. Maximum 3 g/day.
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.
No specific dose adjustment; monitor renal function and adjust based on Cr Cl. Caution with concurrent nephrotoxic agents.
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 FDA black box warning.
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.
Hypersensitivity reactions including anaphylaxis have been reported; contraindicated in patients with known penicillin allergy.,Clostridium difficile-associated diarrhea (CDAD) may occur and must be considered in patients presenting with diarrhea after antibiotic use.,Serious skin reactions (e.g., Stevens-Johnson syndrome, toxic epidermal necrolysis) can occur; discontinue if rash or other allergic signs appear.,Use caution in patients with renal impairment; dosage adjustment may be necessary.,Prolonged use may result in superinfection with non-susceptible organisms.
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
History of hypersensitivity reaction to any penicillin or beta-lactam antibiotic.,Infectious mononucleosis (increases risk of maculopapular rash).,Phenylketonuria (some formulations contain aspartame).
Hypersensitivity to ampicillin or any penicillin,Hypersensitivity to cephalosporins (cross-allergenicity possible),Infections caused by penicillinase-producing bacteria (including methicillin-resistant Staphylococci)
No significant food interactions. Absorption is not affected by food; may be taken with meals to reduce gastrointestinal upset. Avoid concurrent alcohol consumption as it may increase risk of side effects like nausea and vomiting.
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.
FDA Pregnancy Category B. No evidence of teratogenicity in animal studies. First trimester: no increased risk of major malformations observed in large cohort studies. Second and third trimesters: use only if clearly needed; no known fetal harm, but caution due to maternal physiological changes.
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.
Amoxicillin is excreted into breast milk in small amounts (M/P ratio approximately 0.014-0.015). Considered compatible with breastfeeding; potential for diarrhea or allergic sensitization in infant, but generally safe.
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.
No dose adjustment required for amoxicillin in pregnancy; however, increased renal clearance and expanded plasma volume may lower serum concentrations. For severe infections, consider standard dosing with monitoring of clinical response.
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.
For streptococcal pharyngitis, amoxicillin 50 mg/kg once daily (max 1 g) is as effective as multiple daily doses and improves adherence. In penicillin-allergic patients, the cross-reactivity risk with cephalosporins is low; a cephalosporin can be used if no history of immediate-type hypersensitivity. Amoxicillin is not effective against penicillinase-producing staphylococci or most Gram-negative organisms due to beta-lactamase production. Monitor for rash in patients with infectious mononucleosis (ampicillin rash occurs more frequently, but amoxicillin also has increased risk). Dose adjustment needed for creatinine clearance <30 m L/min.
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.
Take exactly as prescribed; complete the full course even if you feel better.,Can be taken with or without food; if stomach upset occurs, take with a meal.,Swallow capsules whole; do not crush or chew; oral suspension shake well before each dose.,Skip missed dose if almost time for next; do not double dose.,Seek immediate medical help for signs of allergic reaction: hives, difficulty breathing, swelling of face/lips/tongue.,May cause diarrhea; contact doctor if watery or bloody stools.,Inform doctor if you are pregnant, planning to become pregnant, or breastfeeding.,Avoid large amounts of grapefruit juice as it may affect absorption (limited clinical significance).
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.
"Amoxicillin may reduce the metabolism of Indinavir via inhibition of CYP3A4, leading to increased plasma concentrations of Indinavir. This can elevate the risk of Indinavir-related toxicities such as nephrolithiasis, hepatotoxicity, and gastrointestinal intolerance. Patients may experience exacerbated adverse effects without a corresponding increase in antiviral efficacy."
"Amoxicillin may inhibit the CYP3A4-mediated metabolism of nicardipine, a calcium channel blocker, leading to increased plasma concentrations of nicardipine. This can potentiate vasodilation and negative chronotropic effects, resulting in an increased risk of hypotension, bradycardia, and peripheral edema. Patients, especially those with pre-existing cardiovascular conditions, should be monitored for enhanced antihypertensive effects and adverse reactions when these drugs are coadministered."
"Amoxicillin may inhibit the metabolism of bortezomib through competitive inhibition of cytochrome P450 enzymes, particularly CYP3A4 and CYP2C19, potentially leading to increased bortezomib exposure. This interaction could result in enhanced toxicity of bortezomib, including peripheral neuropathy, myelosuppression, and gastrointestinal adverse effects. Clinicians should monitor for signs of bortezomib toxicity when amoxicillin is coadministered, especially in patients with pre-existing hepatic impairment or other risk factors."
"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."
Explore head-to-head clinical comparisons of other medications in the same therapeutic classes.
Common clinical questions about AMOXICILLIN vs Ampicillin, answered by our medical review team.
AMOXICILLIN is a Penicillin Antibiotic that works by Amoxicillin is a beta-lactam antibiotic that inhibits bacterial cell wall synthesis by binding to penicillin-binding proteins (PBPs), inhibiting transpeptidase activity, and activating autolytic enzymes.. 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.. They differ in pharmacokinetic profiles, FDA-approved indications, and side effect profiles.
Potency comparisons between AMOXICILLIN and Ampicillin 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 AMOXICILLIN is: 250-500 mg orally every 8 hours or 500-875 mg orally every 12 hours; for severe infections, up to 1 g orally every 8 hours.. The standard adult dose of Ampicillin is: 250-500 mg orally every 6 hours; 1-2 g IV/IM every 4-6 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 AMOXICILLIN and Ampicillin 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. AMOXICILLIN is classified as Category A/B. FDA Pregnancy Category B. No evidence of teratogenicity in animal studies. First trimester: no increased risk of major malformations observed in large cohort studies. Second and th. 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. Always consult a maternal-fetal medicine specialist before taking either drug during pregnancy or lactation.