ampicillin
penicillin antibiotic· Brands: Principen, Ampisyn, AmpicinIMIVPO
Mechanism of action
Ampicillin is a beta-lactam antibiotic that binds to penicillin-binding proteins (PBPs) on the bacterial cell wall, inhibiting the transpeptidation step of peptidoglycan synthesis. This results in defective cell wall formation and ultimately bacterial lysis. It is a broad-spectrum penicillin with activity against many Gram-positive and some Gram-negative organisms, but it is susceptible to beta-lactamases, so it is often combined with a beta-lactamase inhibitor such as sulbactam.
Pharmacokinetics
Ampicillin is acid-stable and can be given orally, though oral bioavailability is approximately 40-50% and is decreased by food. It is widely distributed in body fluids and tissues, with adequate penetration into cerebrospinal fluid (CSF) when the meninges are inflamed. It crosses the placenta and enters breast milk. The drug is metabolised to a small extent and the majority is excreted unchanged in the urine. The elimination half-life is around 1 hour, but it is prolonged in renal impairment.
Indications & dosing
Community-acquired pneumonia
For penicillin-susceptible organisms; adjust for severe infections.
- adult · PO: 500–1000 mg 6-hourly · For penicillin-susceptible organisms; adjust for severe infections.
Urinary tract infections
When susceptibility is confirmed.
- adult · PO: 500 mg 6-hourly · When susceptibility is confirmed.
Bacterial meningitis
For susceptible infections; usually combined with other antibiotics.
- adult · IV: 2000 mg 4-hourly · For susceptible infections; usually combined with other antibiotics.
Infective endocarditis
In combination with gentamicin for enterococcal endocarditis.
- adult · IV: 2000 mg 4-hourly · In combination with gentamicin for enterococcal endocarditis.
Acute otitis media in children
Dose per day divided into four doses.
- pediatric · PO: 25–50 mg/kg/day divided 6-hourly · Dose per day divided into four doses.
Contraindications & precautions
- Hypersensitivity to penicillin antibiotics · Includes history of anaphylaxis, urticaria, or other immediate reactions to penicillins.
- Hypersensitivity to any beta-lactam antibiotic · Cross-allergy may occur with cephalosporins, carbapenems, and monobactams.
- Infectious mononucleosis · Increased risk of skin rash; avoid if possible.
- Renal impairment · Dose adjustment may be required; monitor renal function.
- Ask about previous penicillin or beta-lactam allergy before prescribing.
- Monitor renal function and adjust dose in renal impairment.
- Use caution in patients with asthma or other allergic conditions.
- Consider the risk of Clostridioides difficile infection if diarrhoea occurs.
- Be aware of possible superinfection with prolonged use.
Adverse effects
- Diarrhoea
- Nausea
- Skin rash
- Anaphylaxisserious
- Pseudomembranous colitisserious
- Blood dyscrasias (e.g., neutropenia, thrombocytopenia)serious
Renal & hepatic adjustment
- eGFR 30+: No dose adjustment required.
- eGFR 10–29: Extend the dosing interval to 8-12 hours.
- eGFR 0–9: Extend the dosing interval to 12-24 hours; consider using an alternative antibiotic.
Monitoring, pregnancy & toxicity
- Renal function · Baseline and periodically during therapy
- Signs of hypersensitivity reaction · Throughout therapy
- Liver function tests · In patients with severe hepatic disease or prolonged therapy
- Complete blood count · If prolonged therapy
- Pregnancy: Ampicillin is generally considered safe during pregnancy; it crosses the placenta but is not known to cause harm. It is commonly used for indications such as urinary tract infections and group B streptococcus prophylaxis.
- Toxicity: Allergic reactions including rash, urticaria and anaphylaxis · Discontinue ampicillin and treat anaphylaxis with adrenaline and supportive measures per local protocol.
- Toxicity: Gastrointestinal symptoms such as nausea, vomiting and diarrhoea · Symptomatic treatment, ensure adequate hydration. Rule out antibiotic-associated colitis.
- Toxicity: Neurotoxicity including seizures and encephalopathy, particularly in renal impairment or with high doses · Discontinue the drug and consider haemodialysis as ampicillin is dialysable.
- Toxicity: Crystalluria leading to acute kidney injury (rare) · Stop the drug, maintain good hydration, and consider haemodialysis if severe.
Clinical notes
Ampicillin is used for respiratory tract infections, urinary tract infections, enterococcal endocarditis, and meningitis (e.g., Listeria monocytogenes). It is inactivated by beta-lactamases, so it is often combined with sulbactam to extend its spectrum. Hypersensitivity reactions, including maculopapular rash, are common, especially in patients with infectious mononucleosis. Cross-allergy exists among penicillins and other beta-lactams. Dose adjustment is required in renal impairment. Always consult local guidelines for susceptibility patterns and dosing.
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BNF; local formulary