An extended-spectrum penicillin antibiotic given by injection
Mezlocillin
An older broad-acting penicillin antibiotic, given by injection in hospital, now largely replaced by newer medicines.
What is Mezlocillin?
Mezlocillin is an extended-spectrum penicillin antibiotic, given by injection into a vein or muscle, that was used in hospital to treat serious infections, including some caused by harder-to-treat bacteria such as Pseudomonas. Because it is a penicillin, the most important safety point is the risk of an allergic reaction, which can rarely be severe (anaphylaxis), so it must not be given to anyone allergic to penicillins. It also carries a high sodium (salt) load, which matters for people with heart or kidney problems. It is now largely obsolete and has been replaced by newer antibiotics.
Education and reference only. This is a plain-language guide to Mezlocillin — 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
Mezlocillin is an antibiotic from the penicillin family, in a group known as extended-spectrum or anti-pseudomonal penicillins because they act against a broad range of bacteria, including some difficult hospital germs such as Pseudomonas. It is given by injection, usually into a vein, in a hospital setting rather than as tablets at home. It was used for serious infections such as those affecting the blood, abdomen, urinary tract or chest. Today it is considered largely obsolete in the UK and has been overtaken by newer, better-studied antibiotics.
How it works
Like other penicillins, mezlocillin works by stopping bacteria from building their cell walls properly. Without a sound wall, the bacteria cannot hold themselves together and are killed, which clears the infection. Its extended spectrum means it reaches a wider range of bacteria than older penicillins, including some that cause serious hospital infections. Because it is broken down and removed by the body fairly quickly, it is given as repeated injections to keep enough of the medicine in the bloodstream while the infection is being treated.
Company & origin
Originated / developed by: Generic (largely obsolete).
An older injectable penicillin antibiotic, now rarely used in the UK, developed to tackle a wide range of bacteria including some difficult hospital ones.
Practical use
How to take Mezlocillin
General, dose-free guidance — always follow your prescriber's and the leaflet's specific instructions.
- It is given as an injection by healthcare staff, usually into a vein, in a hospital setting.
- Always tell the team about any allergy to penicillins or other antibiotics before it is given.
- Complete the full course as advised, even if you feel better, so the infection is fully treated.
- Mention any heart, blood-pressure or kidney problems, as the medicine carries a high salt load.
- Report any rash, swelling, itching or difficulty breathing straight away, as these can signal an allergic reaction.
Weighing it up
Advantages & disadvantages of Mezlocillin
Advantages
- Acts against a broad range of bacteria, including some difficult hospital germs such as Pseudomonas.
- Can be used for serious infections where a wide-acting antibiotic is needed.
- Part of the well-understood penicillin family, with a long history of use.
Disadvantages
- Carries the penicillin risk of allergic reactions, which can rarely be severe.
- Has a high sodium (salt) load, which can be a problem for people with heart or kidney conditions.
- Now largely obsolete and replaced by newer antibiotics, so rarely used today.
Practical use
Good to know
The single most important thing about mezlocillin, as with any penicillin, is allergy: anyone who has had a serious reaction to a penicillin must not be given it, because reactions can rarely be life-threatening (anaphylaxis), and any rash, swelling or difficulty breathing needs urgent attention. A second practical point is that it carries a high sodium (salt) load, which can matter for people with heart failure, high blood pressure or kidney problems, where extra salt and fluid are a concern. Like all antibiotics, it should be used only when truly needed, as overuse encourages resistant bacteria. It is a hospital injection rather than a home medicine, and because it is now largely obsolete, you are far more likely to be offered a newer antibiotic instead.
Who should not take it / use with caution
- People who have had a serious allergic reaction to penicillins must not be given it.
- It is used with caution in people with heart failure or kidney problems because of its high salt load.
- It is used with care, and dose-adjusted by the team, in people with reduced kidney function.
Monitoring
- Watching for allergic reactions, especially when it is first given.
- Checking kidney function and salt and fluid balance during treatment.
- Reviewing how the infection responds and checking blood counts with longer courses.
Side effects
- Allergic reactions, ranging from rash to, rarely, severe whole-body reactions (anaphylaxis).
- Diarrhoea, nausea or other stomach upset, as with many antibiotics.
- Effects linked to its salt content, such as fluid retention, especially in heart or kidney problems.
- Less commonly, changes in blood tests, including effects on blood cells with longer use.
Key interactions
- It may interact with other medicines that affect the kidneys or salt and fluid balance.
- Some antibiotics work less well together, so it is combined with others only on specialist advice.
- Always tell the team about all your medicines, as penicillins can affect certain other treatments.
Available as: A powder made up into a solution for injection into a vein or muscle.
Answers
Mezlocillin: frequently asked questions
What is mezlocillin used for?
It is an older injectable penicillin antibiotic that was used in hospital for serious infections, including some caused by harder-to-treat bacteria such as Pseudomonas. It is now largely obsolete.
Why does penicillin allergy matter so much?
Mezlocillin is a penicillin, so anyone allergic to penicillins must not be given it, because reactions can rarely be severe and life-threatening (anaphylaxis).
Why is its salt content a concern?
It carries a high sodium (salt) load, which can cause fluid retention and is a particular concern for people with heart failure or kidney problems.
Is it taken as tablets?
No. It is given by injection, usually into a vein, by healthcare staff in a hospital setting rather than as tablets at home.
Why is it rarely used now?
It has been overtaken by newer, better-studied antibiotics, so it is considered largely obsolete in the UK today.
Authoritative sources
- BNF
- NICE CKS
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