OMNIPEN (AMPICILLIN)
Clinical safety rating: safe
Probenecid decreases renal tubular secretion of ampicillin Allopurinol may increase the incidence of skin rashes Serious and occasionally fatal hypersensitivity reactions have been reported.
Inhibits bacterial cell wall synthesis by binding to penicillin-binding proteins (PBPs), inhibiting transpeptidation and peptidoglycan cross-linking.
| Metabolism | Partially hepatic; primarily eliminated renally as unchanged drug. |
| Excretion | Renal excretion accounts for approximately 90% of elimination, primarily via tubular secretion and glomerular filtration. Biliary/fecal excretion is minimal, <10%. |
| Half-life | Terminal elimination half-life is approximately 1-1.5 hours in adults with normal renal function. In neonates, it may be prolonged to 2-4 hours; in renal impairment, half-life can extend significantly (up to 8-20 hours in severe impairment). |
| Protein binding | Approximately 17-20% bound to serum albumin. |
| Volume of Distribution | 0.2-0.3 L/kg, indicating limited distribution to extracellular fluid and low tissue penetration, except in inflamed meninges where penetration increases. |
| Bioavailability | Oral: 30-50% (due to acid lability and incomplete absorption); IM: approximately 80-90%. |
| Onset of Action | Oral: 1-2 hours; IV: immediate (within minutes); IM: within 30-60 minutes. |
| Duration of Action | Oral: 6-8 hours; IV/IM: 4-6 hours. Clinical duration is dose-dependent and influenced by renal function. |
250-500 mg orally every 6 hours; 500 mg to 2 g intramuscularly or intravenously every 4-6 hours.
| Dosage form | CAPSULE |
| Renal impairment | CrCl 30-50 mL/min: administer every 6-8 hours. CrCl 10-30 mL/min: administer every 8-12 hours. CrCl <10 mL/min: administer every 12-16 hours. |
| Liver impairment | No dose adjustment required for Child-Pugh class A, B, or C. |
| Pediatric use | Neonates: 50 mg/kg/day divided every 8 hours. Infants and children: 50-100 mg/kg/day orally divided every 6 hours; 100-200 mg/kg/day intravenously divided every 6 hours. |
| Geriatric use | Monitor renal function; adjust dose based on creatinine clearance as per adult renal adjustment guidelines. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Probenecid decreases renal tubular secretion of ampicillin Allopurinol may increase the incidence of skin rashes Serious and occasionally fatal hypersensitivity reactions have been reported.
| FDA category | Human |
| Breastfeeding | Ampicillin is excreted in breast milk in small amounts (M/P ratio approximately 0.1-0.2). Considered compatible with breastfeeding by the American Academy of Pediatrics. Theoretical risk of diarrhea or allergic reaction in infant; monitor for rash, diarrhea, or candidiasis. |
| Teratogenic Risk |
■ FDA Black Box Warning
No FDA boxed warning.
| Common Effects | Diarrhea |
| Serious Effects |
["Known hypersensitivity to penicillins","Infections caused by penicillinase-producing organisms"]
| Precautions | ["Hypersensitivity reactions (including anaphylaxis) in penicillin-allergic patients","Superinfection with prolonged use","Clostridioides difficile-associated diarrhea","Hematologic abnormalities (e.g., neutropenia, thrombocytopenia) with high doses or prolonged therapy","Cautious use in renal impairment (dose adjustment needed)"] |
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| FDA Pregnancy Category B. Ampicillin crosses the placenta. Animal studies reveal no evidence of harm to the fetus. No well-controlled studies in pregnant women; however, it is widely used and considered low risk. No known teratogenic effects in any trimester. |
| Fetal Monitoring | No specific monitoring required beyond standard prenatal care. Monitor for maternal allergic reactions (rash, anaphylaxis), gastrointestinal side effects, and signs of superinfection. In neonates, no specific monitoring needed unless maternal use near delivery. |
| Fertility Effects | No known adverse effects on fertility in animal studies or human reports. Ampicillin does not affect spermatogenesis or ovulation. |