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. |
| Molecular Weight | 349.41 Da |
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 | Ampicillin crosses the placenta. Animal studies have not shown fetal harm; however, inadequate human studies exist. Use only if clearly needed. |
| 2nd trimester | Ampicillin is generally considered safe in the second trimester when clinically indicated. No known teratogenicity. |
| 3rd trimester | Ampicillin is safe in the third trimester for treatment of infections (e.g., GBS prophylaxis). Potential for altered gut flora in neonate. |
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 |
| Placental transfer | Ampicillin readily crosses the placenta; therapeutic fetal serum concentrations are achieved within 1 hour of maternal administration. |
■ FDA Black Box Warning
No FDA boxed warning.
| Common Effects | Diarrhea |
| Serious Effects |
Hypersensitivity to ampicillin or any penicillinInfections 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) |
| Food/Dietary | Food decreases absorption; take on an empty stomach. Avoid alcohol as it may increase adverse effects. |
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| Breastfeeding | Ampicillin is excreted into breast milk in low concentrations (less than 0.1% of maternal dose). Considered compatible with breastfeeding; adverse effects in nursing infants are rare, but monitor for diarrhea, rash, or candidiasis. |
| Lactation Rating | L1 (Safe) |
| Teratogenic Risk | 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. |
| Clinical Pearls | Ampicillin is a beta-lactam antibiotic susceptible to beta-lactamase inactivation; ineffective against penicillinase-producing staphylococci. Check for cross-allergy in penicillin-allergic patients. Adjust dose in renal impairment: CrCl <30 mL/min: administer every 12 hours; CrCl <10 mL/min: every 24 hours. Common adverse effects: diarrhea, rash (especially in patients with mononucleosis). |
| Patient Advice | Take ampicillin exactly as prescribed, even if you feel better. · Finish the full course of treatment; do not stop early. · Take on an empty stomach (1 hour before or 2 hours after meals) for best absorption. · Shake the oral suspension well before each use and measure with a proper dosing device. · Report any rash, severe diarrhea, or signs of allergic reaction (difficulty breathing, swelling) immediately. |