Ampicillin

Ampicillin tablets are penicillin-class (beta-lactam) antibiotics used to treat certain bacterial infections when the bacteria are susceptible. Use them exactly as prescribed - antibiotics do not work for colds, flu or other viral illnesses.

Antimicrobial action

Ampicillin is a beta-lactam antibiotic. It is bactericidal and works in a similar way to penicillin G (benzylpenicillin). As an aminopenicillin with an amino group side chain attached to the basic penicillin structure, ampicillin is better able to penetrate the outer membrane of some Gram-negative bacteria and has a broader spectrum of activity.

Spectrum of activity

Ampicillin is similar to benzylpenicillin in its action against Gram-positive organisms, including Streptococcus pneumoniae and other streptococci, but, with the possible exception of activity against Enterococcus faecalis, it may be slightly less potent than benzylpenicillin. Listeria monocytogenes is generally susceptible. The Gram-negative cocci Moraxella catarrhalis (Branhamella catarrhalis), Neisseria gonorrhoeae, and N. meningitidis may be susceptible, depending on local resistance patterns.

Ampicillin is more active than benzylpenicillin against some Gram-negative bacilli, including Haemophilus influenzae and Enterobacteriaceae such as Escherichia coli, Proteus mirabilis, Salmonella and Shigella spp. It is inactive against Pseudomonas aeruginosa.

Ampicillin also has activity similar to benzylpenicillin against other organisms, including many anaerobes and Actinomyces spp.

Activity with other antimicrobials. Synergy has been shown between ampicillin and aminoglycosides against some organisms, including enterococci. Variable effects, ranging from synergy to antagonism, have been reported between ampicillin and other beta-lactams, bacteriostatic drugs such as chloramphenicol, and rifampicin.

Resistance. Like benzylpenicillin, ampicillin is inactivated by beta-lactamases, although other mechanisms may be responsible for resistance in some species. Resistance rates vary geographically; many strains of staphylococci and some Gram-negative organisms may be resistant.

Pharmacokinetics

Ampicillin is relatively resistant to inactivation by gastric acid and is moderately well absorbed from the gastrointestinal tract after oral doses. Food can interfere with the absorption of ampicillin, so doses are often taken on an empty stomach, for example at least 30 minutes before meals. Peak concentrations in plasma are reached in about 1 to 2 hours; after a 500-mg oral dose, peak levels have been reported to range from 3 to 6 mcg/mL.

Ampicillin is widely distributed and crosses the placenta; small amounts pass into breast milk. There is little diffusion into the CSF except when the meninges are inflamed. About 20% is bound to plasma proteins, and the plasma half-life is about 1 to 1.5 hours, but this may be increased in neonates, older adults, and patients with renal impairment.

Ampicillin is metabolised to some extent to penicilloic acid and is eliminated mainly by the kidneys. Renal clearance occurs partly by glomerular filtration and partly by tubular secretion, and is reduced by probenecid.

Uses

Ampicillin tablets are used to treat a range of infections caused by susceptible organisms. Whenever possible, treatment in the UK should be guided by culture and susceptibility results.

Ampicillin vs amoxicillin

How to take

Take ampicillin tablets exactly as prescribed. Because food can reduce absorption, they are often taken on an empty stomach, for example at least 30 minutes before meals. Finish the full course unless your prescriber tells you to stop.

Side effects

Common side effects may include diarrhoea, nausea, vomiting and skin rash. In UK medical practice, seek urgent medical help if you develop signs of a severe allergic reaction, such as swelling of the face or throat or trouble breathing, severe skin reactions, or severe or persistent diarrhoea, which can be a sign of Clostridioides difficile-associated diarrhoea.

Pregnancy and breastfeeding

Pregnancy: animal studies with penicillins have not shown harm to the fetus, but there are no adequate, well-controlled studies in pregnant women. Use during pregnancy only if clearly needed and as prescribed.

Breastfeeding: ampicillin-class antibiotics are excreted in breast milk and may cause sensitisation in the infant; use with caution and contact a clinician if the baby develops a rash, diarrhoea or thrush.

Administration in renal impairment

Dose adjustment may be needed in renal impairment. Follow your prescriber's instructions if you have kidney disease or are receiving dialysis.

Storage

Store at 20°C to 25°C (68°F to 77°F). Keep the container tightly closed and out of the reach of children.