An older anti-pseudomonal penicillin antibiotic
Carbenicillin
An older penicillin antibiotic, once used for pseudomonas infections, now largely obsolete.
What is Carbenicillin?
Carbenicillin is an older penicillin antibiotic that was once used to treat difficult infections, particularly those caused by a bacterium called pseudomonas. It works like other penicillins by attacking the bacterial cell wall. It carries the usual penicillin concern of allergy, and because it contains a lot of sodium it can be a problem for people who need to limit salt, such as those with heart or kidney problems. It can also affect blood platelets and increase the risk of bleeding. It is largely obsolete today and has been replaced by newer antibiotics in the UK.
Education and reference only. This is a plain-language guide to Carbenicillin — it deliberately contains no doses. Doses depend on the person, the brand and the reason for treatment, and belong with your prescriber. Always check the BNF, the product labelling (SmPC) and follow medical advice.
What it is
Carbenicillin is a penicillin antibiotic that was developed to tackle harder-to-treat infections, especially those caused by Pseudomonas aeruginosa, a bacterium that resists many ordinary antibiotics. It was given by injection or infusion, usually in hospital. Like all penicillins it works by damaging the bacterial cell wall. It is now considered largely obsolete: newer and better-tolerated antibiotics have taken its place, and it is rarely if ever used in current UK practice. It is described here mainly for completeness and historical context.
How it works
Carbenicillin, like other penicillins, works by blocking the bacteria's ability to build and repair their cell wall. Without a strong wall the bacteria cannot hold their shape and burst, which clears the infection. Its particular feature was activity against pseudomonas and some other resistant bacteria that ordinary penicillins could not reach. To achieve useful levels in the body it had to be given in large amounts, which is why its high sodium content and its effects on blood platelets became important practical drawbacks compared with later antibiotics.
Company & origin
Originated / developed by: Various (largely historical).
An older penicillin antibiotic once used against difficult infections such as pseudomonas; it is now largely obsolete and rarely, if ever, used in the UK.
Practical use
How to take Carbenicillin
General, dose-free guidance — always follow your prescriber's and the leaflet's specific instructions.
- It is given by injection or infusion in hospital under medical supervision, not taken at home.
- Tell the team about any allergy to penicillin or other antibiotics before it is given.
- Mention any heart, blood-pressure or kidney problems, as its high salt content can matter.
- Report any unusual bruising or bleeding, as it can affect blood platelets.
- Complete any course as directed, although in practice newer antibiotics are now used instead.
Weighing it up
Advantages & disadvantages of Carbenicillin
Advantages
- Was effective against difficult bacteria, including pseudomonas, that ordinary penicillins could not treat.
- Worked by the well-understood penicillin action of attacking the bacterial cell wall.
- Filled an important gap before better anti-pseudomonal antibiotics became available.
Disadvantages
- Carries a high sodium (salt) load, which can be a problem for people with heart or kidney problems.
- Can affect blood platelets and increase the risk of bleeding.
- Largely obsolete, with safer and more effective antibiotics now used instead.
Practical use
Good to know
Because carbenicillin is a penicillin, the most important safety point is penicillin allergy: anyone who has had a serious allergic reaction to a penicillin should not receive it. Two features set it apart from everyday penicillins. First, it carries a high sodium (salt) load, because large amounts were needed, which can cause problems for people with heart failure, high blood pressure or kidney problems who need to limit salt and fluid. Second, it can interfere with blood platelets and the clotting process, increasing the risk of bleeding or bruising, which needed watching during treatment. These drawbacks, along with the arrival of better antibiotics, are why it is no longer in routine use. As with any antibiotic, it can upset the gut and, rarely, cause a serious bowel infection (Clostridioides difficile).
Who should not take it / use with caution
- People who have had a serious allergic reaction to penicillin should not receive it.
- It is used with great caution, or avoided, in people who need to limit salt, such as those with heart failure or severe kidney problems.
- It is used with caution in people with bleeding problems, because it can affect platelets.
- In practice it is no longer in routine use, having been replaced by newer antibiotics.
Monitoring
- Checking for penicillin allergy before it is given.
- Watching for bleeding or bruising and, where needed, blood clotting tests.
- Monitoring salt and fluid balance in people with heart or kidney problems.
Side effects
- Allergic reactions, ranging from rashes to, rarely, severe reactions.
- Problems linked to its high salt content, such as fluid retention, in people who are vulnerable.
- Bleeding or bruising from its effect on blood platelets.
- Diarrhoea or stomach upset and, rarely, a serious bowel infection (Clostridioides difficile).
Key interactions
- Caution with other medicines that affect bleeding or clotting, as carbenicillin can add to the risk.
- Its high sodium content can matter alongside medicines for heart failure or high blood pressure.
- As with other penicillins, tell the team about all your medicines and any antibiotic allergies.
Available as: A powder made up into a solution for injection or infusion (historical use).
Answers
Carbenicillin: frequently asked questions
What was carbenicillin used for?
It was an older penicillin antibiotic used for difficult infections, particularly those caused by pseudomonas; it is now largely obsolete and replaced by newer antibiotics.
Why does its salt content matter?
Carbenicillin had to be given in large amounts and carries a high sodium (salt) load, which can cause problems for people with heart failure, high blood pressure or kidney problems.
Can people with penicillin allergy take it?
No. As a penicillin, it should not be given to anyone who has had a serious allergic reaction to a penicillin.
Does it affect bleeding?
Yes, it can interfere with blood platelets and the clotting process, increasing the risk of bruising or bleeding, which needs watching.
Is it still used?
It is largely obsolete and rarely if ever used in current UK practice, as safer and more effective antibiotics are now available.
The wider class
About Penicillin antibiotic (anti-pseudomonal)
Carbenicillin belongs to the penicillin antibiotic (anti-pseudomonal) class. For how the class as a whole works, its shared safety principles and monitoring, see the full guide.
Browse by body system
Authoritative sources
- BNF
- NICE CKS
Building a medicines information resource?
We create evidence-led, dose-free drug and formulary references for teams.