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Dr Karen Martin
Reviewed by Dr Karen MartinReviewed on 19.10.2023 | 2 minutes read

Penicillins are a class of antibiotic medications usually used to treat bacterial infections caused by gram-positive bacteria (like staphylococcus and streptococcus that can cause skin and throat infections, amongst many others). Commonly used penicillin antibiotics include amoxicillin, flucloxacillin, and phenoxymethylpenicillin (penicillin V). Many more are used to treat severe life-threatening infections like meningitis.

Only 0.05% or less of the population may experience an anaphylactic reaction. An allergy to one type of penicillin means you are likely to be allergic to other penicillins as they are related. Always let your prescriber know if you have any allergies to any medications and what reaction you experience. Penicillins are only available on prescription to ensure antimicrobial stewardship.

Who are penicillins for?

Penicillins are often the first choice to treat bacterial infections where possible compared to other antibiotic classes because they have a narrow therapeutic spectrum. Penicillins treat susceptible bacterial infections, which can cause skin and respiratory infections. Adults, pregnant women, and children can be treated with penicillin.

You can help prevent the development and spread of antibiotic resistance by only taking prescribed and directed antibiotics. Taking antibiotics unnecessarily can increase the risk of the naturally occurring bacteria in your body developing resistance.

How do penicillins work?

Penicillins work by interfering with the production of the bacterial cell wall, which causes the bacteria to die. It cannot be used to treat viral infections that cause colds and flu. Some strains of bacteria are resistant to penicillin antibiotics because they produce an enzyme that breaks down the antibiotic before it can work. You may have to take it with the enzyme inhibitor to work around this (e.g., co-amoxiclav = amoxicillin and clavulanic acid).

Should anyone avoid taking it?

Penicillins should not be used if you have ever been allergic to any penicillin antibiotic. A true penicillin allergy can cause a raised red rash (hives), itchy skin, wheezing or trouble breathing. This is different from experiencing side effects, including gastrointestinal upset (nausea, diarrhea). In mild allergies, penicillin may still be prescribed if it is deemed that the benefits (treating a life-threatening infection) outweigh the risks (mild allergic rash).

Are there any side effects?

Penicillins may cause gastrointestinal upset (nausea, vomiting, diarrhea, abdominal pain). These side effects are short-lived and will pass once the course of antibiotics is finished.

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This article has been written by UK-based doctors and pharmacists, so some advice may not apply to US users and some suggested treatments may not be available. For more information, please see our T&Cs.
Dr Karen Martin
Reviewed by Dr Karen Martin
Reviewed on 19.10.2023
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