Anti-infective
Penicillins
Beta-lactam antibiotics — A large, widely used antibiotic family — first-line for many common infections.
Education and reference only. This is a plain-language class overview — it deliberately contains no doses. Always check the current Summary of Product Characteristics (SmPC), the BNF and your local formulary before prescribing or administering any medicine.
What it is
Penicillins are one of the oldest and most useful antibiotic families, used for chest, throat, urinary, skin and dental infections among many others. Amoxicillin and similar agents are everyday first-line choices.
How it works
They block the enzymes bacteria use to build their cell wall, causing the wall to fail and the bacterium to die. Adding a beta-lactamase inhibitor (as in co-amoxiclav) extends their reach against resistant organisms.
In practice
In practice the most important step is clarifying the allergy history: a true penicillin allergy (rash, swelling, anaphylaxis) is different from a side-effect like nausea, and mislabelling drives patients onto broader, less ideal antibiotics. Choose the narrowest effective agent, follow local guidance and stewardship, and adjust the dose in significant renal impairment.
Examples
Practical use
How to take it & use it well
- Take penicillins such as amoxicillin or flucloxacillin as prescribed and complete the whole course, even once you feel well.
- Flucloxacillin is best taken on an empty stomach, a while before food, while amoxicillin can be taken with or without food.
- Space the doses evenly through the day so the level in your body stays steady.
- If you miss a dose, take it when you remember unless the next is almost due, then skip it; do not take a double dose.
- Tell your prescriber straight away if you have ever reacted to penicillin before, as this is an important safety check.
- Stop and seek urgent help if you develop a rash, swelling of the face or throat, or breathing difficulty.
Common uses
- Respiratory, ear, throat and dental infections
- Skin and soft-tissue infections (flucloxacillin)
- Urinary and other infections per local guidance
Monitoring
- Allergy status before prescribing
- Response to treatment and stewardship review
- Renal function where relevant
Weighing it up
Advantages & disadvantages
Advantages
- They are effective, well-established antibiotics for many common bacterial infections.
- They have a long safety record and are suitable for many people, including in pregnancy when appropriate.
- They are generally well tolerated by those who are not allergic.
- They come in tablet, capsule and liquid forms, so they suit a range of ages.
- They are inexpensive and widely available.
Disadvantages
- Allergy is the main concern and can range from a rash to a rare but serious whole-body reaction.
- They commonly cause diarrhoea, nausea and stomach upset.
- They do not work against viral illnesses such as colds and flu.
- Overuse adds to antibiotic resistance, so they should be used only when needed.
- Some people develop thrush after a course because the antibiotic disturbs the body's natural balance of microbes.
Key safety principles
What to watch for
- Allergy is important — clarify whether a reported "allergy" is a true allergy or an intolerance.
- Cross-reactivity with cephalosporins is lower than once thought but still considered in severe allergy.
- Reduce dose in significant renal impairment; co-amoxiclav carries a small risk of liver effects.
Key interactions
What to avoid or check alongside
- They can increase the effect of blood thinners, raising bleeding risk, so monitoring may be needed.
- A medicine used for gout can raise penicillin levels in the body, which is sometimes used deliberately but otherwise noted.
- Antibiotics can make some other treatments less reliable, so mention everything you take, including the contraceptive pill if you have sickness or diarrhoea.
- Combining with certain other antibiotics is sometimes avoided as they may work against each other.
- Always tell your prescriber about previous reactions to any penicillin or related antibiotic before starting.
Patient & carer advice
- Tell us about any previous reaction to antibiotics and what happened
- Complete the course as advised
- Report rash, swelling or breathing difficulty urgently
Use with
Related clinical calculators
Dose and risk decisions for this class often depend on renal function, weight or bleeding/stroke risk. These tools help:
Answers
Penicillins: frequently asked questions
How do I know if I am allergic to penicillin?
Signs can include a rash, itching, swelling, wheezing or, rarely, a severe whole-body reaction. If you have reacted before, tell every healthcare professional so it can be avoided.
Do I have to finish the course?
Yes. Finishing the prescribed course gives the best chance of clearing the infection and helps prevent resistance, even once you feel better.
Why does amoxicillin sometimes cause a rash?
A rash can be a sign of allergy, but it can also appear for other reasons such as a viral illness. Any new rash should be checked promptly.
Can I drink alcohol with penicillin?
Moderate alcohol does not usually stop penicillins working, but it is sensible to rest and avoid heavy drinking while you are unwell.
Will it treat my cold?
No. Penicillins treat bacterial infections, not the viruses that cause colds and flu, so they will not help these illnesses.
Authoritative sources
Always verify against the source
This overview is for orientation. For doses, interactions, contra-indications and the full monograph, use:
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