Principen (Bristol-Myers Squibb) — oral capsules, US; largely replaced by generics · Omnipen (Wyeth) — historical US brand; discontinued · Ampicin (various Commonwealth manufacturers) · Penbritin (UK/Commonwealth; historical brand) · Totacillin — historical brand · Ampicillin sodium for injection (multiple generic manufacturers — Pfizer, Sandoz, Fresenius Kabi, Hikma) — standard IV formulation in current use · Unasyn (ampicillin-sulbactam) — combination IV product with beta-lactamase inhibitor; distinct from ampicillin alone
Clinical safety rating: safe
Human studies have proved safety
Ampicillin is a semi-synthetic, acid-stable aminopenicillin that exerts concentration-independent, time-dependent bactericidal activity. It irreversibly acylates and inhibits penicillin-binding proteins (PBPs), particularly PBP-1A, PBP-1B, PBP-2, and PBP-3, which are transpeptidases and carboxypeptidases responsible for cross-linking of peptidoglycan strands in the bacterial cell wall. Inhibition of PBPs prevents cell wall synthesis and triggers autolytic enzymes, leading to cell wall lysis and osmotic death. The aminobenzyl side chain at the 6-APA position extends activity compared to natural penicillins to include gram-negative organisms such as Haemophilus influenzae (beta-lactamase-negative strains), Escherichia coli (susceptible strains), Proteus mirabilis, Salmonella spp., Shigella spp., and importantly Listeria monocytogenes (which lacks a beta-lactamase). Ampicillin retains full activity against Group B Streptococcus (Streptococcus agalactiae), Group A Streptococcus, penicillin-susceptible Streptococcus pneumoniae, Enterococcus faecalis, and Listeria — the four organisms of greatest clinical relevance in obstetric practice. It is hydrolysed by beta-lactamases and therefore inactive against beta-lactamase-producing strains.
| Metabolism | Ampicillin undergoes minimal hepatic metabolism. Approximately 10–20% is converted to the inactive metabolite ampicilloic acid and other penicilloic acid derivatives. The dominant elimination pathway is renal excretion of unchanged drug. After oral administration, first-pass metabolism is negligible, but bioavailability is limited by incomplete absorption (~30–55%). IV ampicillin bypasses this limitation entirely. CYP450 involvement: None. Ampicillin is neither a substrate, inhibitor, nor inducer of CYP450 enzymes, which underlies its low pharmacokinetic drug interaction potential. Bile excretion: A small fraction (~10%) undergoes biliary excretion with limited enterohepatic recirculation, contributing to intestinal flora disruption and GI side effects. Pregnancy-specific metabolic changes: The physiological increase in GFR during pregnancy (~50% above baseline by second trimester) accelerates renal clearance of ampicillin, modestly shortening the half-life and reducing trough concentrations. Expanded plasma volume increases the volume of distribution. These changes support use of higher-frequency dosing intervals (every 4–6 hours rather than every 8 hours) for serious obstetric infections. |
Dosing varies by indication and patient profile. Always follow your institution's current prescribing guidelines.
| Renal impairment | Consult protocols for adjustment. |
| Liver impairment | Consult protocols for adjustment. |
| 1st trimester | Safe. No increased risk of major congenital malformations based on the Collaborative Perinatal Project and multiple large cohort studies. Appropriate for treatment of confirmed susceptible bacterial infections including UTI, skin and soft tissue infections, and dental infections where systemic therapy is required. Oral bioavailability is limited — amoxicillin is preferred for outpatient oral therapy. GI side effects may compound first-trimester nausea. |
| 2nd trimester | Safe. Appropriate for treatment of pyelonephritis (IV), UTI, and other susceptible bacterial infections. Empiric use for UTI requires local susceptibility data — E. coli resistance to ampicillin is high (>40–60% in many regions) and urine culture with susceptibility testing should guide therapy. Standard agent in chorioamnionitis combination regimens if this complication arises at the limits of viability. Listeriosis empiric coverage should always include ampicillin regardless of trimester. |
| 3rd trimester | Safe. First-line IV antibiotic for GBS intrapartum prophylaxis when penicillin G is unavailable (2 g IV load, then 1 g IV every 4 hours until delivery). Essential component of empiric chorioamnionitis treatment (with gentamicin ± clindamycin/metronidazole). Drug of choice (with gentamicin) for listeriosis — critical given Listeria's predilection for immunocompromised states including pregnancy and its cephalosporin-resistance. Neonates born to treated mothers should be monitored for GI colonisation effects but ampicillin use does not require withholding breastfeeding. |
Clinical note
Ampicillin is a broad-spectrum aminopenicillin antibiotic with an established safety record across all trimesters of pregnancy. It is the first-line intravenous agent for Group B Streptococcus (GBS) intrapartum prophylaxis per CDC and ACOG guidelines, the drug of choice (in combination with gentamicin) for treatment of listeriosis in pregnancy, and a standard agent for urinary tract infections, chorioamnionitis regimens, and endocarditis prophylaxis in obstetric patients. No teratogenic signal has been identified in large epidemiological cohorts. It does not carry the necrotising enterocolitis (NEC) risk associated with amoxicillin-clavulanate in PPROM. Oral bioavailability is limited and erratic; the IV/IM route is standard for serious obstetric indications.
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| Placental transfer | Yes — ampicillin crosses the placenta readily by passive diffusion. Cord blood concentrations reach approximately 30–50% of maternal serum concentrations within 1 hour of IV administration, with cord:maternal ratios approaching 1.0 at delivery after repeated dosing. Ampicillin distributes into amniotic fluid — concentrations are lower than maternal serum but sufficient to have therapeutic effects in the fetal compartment. This transplacental transfer is the pharmacological basis for its efficacy in preventing vertical GBS transmission and treating fetal Listeria infection. Fetal tissue concentrations are therapeutically relevant for susceptible organisms in fetal lung, liver, and blood. |
| Breastfeeding | Ampicillin is excreted into breast milk in low concentrations. Reported milk levels are typically 0.1–1.2 mg/L following standard IV dosing. The relative infant dose (RID) is estimated at <1% of the maternal weight-adjusted dose, well below the 10% threshold of clinical concern. The most commonly reported adverse effects in nursing infants are GI disturbance (loose stools, diarrhoea) and candidal diaper rash due to alteration of intestinal flora. Rare allergic sensitisation is theoretically possible. Temporary interruption of breastfeeding is not warranted. Premature infants or neonates with suspected renal impairment may warrant closer observation given reduced drug clearance. |
