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Antibiotics, penicillins

Page last reviewed: 13/07/2011

Penicillins are antibiotics. These are medicines that kill bacteria which cause infection.

They can be used to treat a wide range of bacterial infections, such as tonsillitis and scarlet fever. Antibiotics don't work against diseases caused by viruses, like the common cold, the flu and most coughs.

Penicillins can be used to prevent infections in people who have had rheumatic fever, people who do not have a spleen and people who have sickle cell disease. They can prevent endocarditis (infection of the heart cavity and heart valves) in people undergoing dental procedures who are at risk of getting this infection (because they have heart valve disease, for example).

How they work

Penicillins weaken and rupture the cell walls of bacteria so that the bacteria die. The cell wall is the layer that surrounds cells, such as bacteria, providing protection and structure.


There are many different types of penicillins with different ranges of activity against bacteria. They include:

  • phenoxymethylpenicillin (also called penicillin V) (brand names include Calvepen)
  • amoxicillin (brand names include Amoxil)
  • ampicillin
  • flucloxacillin (brand names include Floxapen)

Some penicillins are combined with other medicines to make combination products such as:

  • co-fluampicil (flucloxacillin plus ampicillin)
  • co-amoxiclav (amoxicillin plus clavulanic acid, a non-antibiotic that enhances the action of penicillins, brand names include Augmentin)

Some penicillins are available only as injections. These are likely to be given in hospital. Examples include:

  • benzylpenicillin (brand name Crystapen)
  • piperacillin plus tazobactam (brand name Tazocin)
  • temocillin (brand name Negaban)
  • ticarcillin plus clavulanic acid (brand name Timentin)

Finishing the course

It is essential to finish taking a prescribed course of antibiotics, even if you feel better, unless a healthcare professional tells you otherwise.

If you stop taking an antibiotic part way through a course, the bacteria can become resistant to the antibiotic. The infection could then be harder to treat in future (see Frequently asked questions about penicillins for more information).

Using Antibiotics properly

It's very important that people take antibiotics properly, and only when they are needed. For example, colds and flu are caused by viruses, so there is no point taking an antibiotic for colds, the flu and most coughs. Read our Action on Antibiotics section to learn more about this.

Page last reviewed: 13/07/2011

When to avoid them

Do not take penicillins if you have had a previous allergic reaction to them.

Patients with a history of allergies, such as asthma, eczema or hay fever, are at higher risk of developing a serious allergic reaction (anaphylactic shock) to penicillins. However, such reactions are rare.

Patients who are allergic to one type of penicillin will be allergic to all (the allergy is related to the basic penicillin structure).

Using them with caution

Take penicillins with caution if you:

  • have severe kidney problems
  • have liver problems
  • are pregnant or breastfeeding (see below)

Also take penicillin with caution if you have had a previous allergic reaction to other beta-lactams (a group of antibiotics that includes penicillins), such as cefadroxil, cefaclor, cefotaxime or cefuroxime.

Pregnancy and breastfeeding

Most penicillins can be used during pregnancy and breastfeeding in the usual doses.

Safe use of phenoxymethylpenicillin during pregnancy has not been definitely established. It should not be used during pregnancy unless clearly needed.

Because phenoxymethylpenicillin passes into breast milk, it should be used with caution in women who are breastfeeding as even small amounts can cause an allergic reaction in a susceptible baby.

Penicillins that are combined with clavulanic acid should not be used during pregnancy and breastfeeding unless absolutely necessary. These penicillins include co-amoxiclav (brand name Augmentin) and tazobactam (brand name Tazocin).

Tell your healthcare professional if you are pregnant or breastfeeding so they can prescribe the most suitable antibiotic for you.

Page last reviewed: 13/07/2011

Some general adverse effects of penicillins are described below. For more specific information on side effects for a particular penicillin

Penicillins do not affect your ability to drive safely.

Common adverse effects

Up to 1 in 10 people may experience the following:

  • nausea (feeling sick)
  • vomiting
  • mild allergic reactions, such as rashes
  • diarrhoea

Rare adverse effects

These include the following:

  • severe allergic reactions, including fever, joint pains, angioedema (swelling in the eyes, lips or tongue) and anaphylaxis (a rare but very serious allergic reaction that causes asthma, throat obstruction and low blood pressure)
  • dizziness
  • hepatitis (liver inflammation)
  • fits (convulsions)
  • antibiotic-associated colitis (inflammation of the large bowel)
  • blood disorders such as leukopenia (a decrease in circulating white blood cells) and thrombocytopenia (low blood platelet count)
  • nephritis (inflammation in the kidneys)
  • thrush (candida) infections

When to get help

If you experience any of the following, stop taking the tablets and tell your doctor immediately:

  • an allergic reaction: symptoms may include shortness of breath, skin rash, itching, swelling of your lips, face or tongue, chills or fever, or painful joints
  • unusual bleeding or bruising
  • seizures (fits)

Page last reviewed: 13/07/2011

When two or more medicines are taken at the same time, the effects of one of the medicines can be altered by the other. This is known as a drug-drug interaction. Penicillins can sometimes interact with other medicines.

Some of the more common interactions are listed below. However, this is not a complete list.

If you want to check that your medicines are safe to take with penicillins, ask your doctor or local pharmacist, or read the patient information leaflet that comes with your medicine.


There is no interaction of alcohol with penicillins.


There is an increased risk of rash when allopurinol is taken with amoxicillin or ampicillin. This is not an allergic reaction.

Combined oral contraceptives

Penicillins may cause your combined oral contraceptive pill to be less effective at preventing pregnancy.

Women taking combined oral contraceptives should use an extra method of contraception (for example, condoms) while taking a short course of penicillins and for seven days after finishing the penicillins. If the period of seven days runs beyond the end of the pill packet into the pill-free break, start a new packet without the usual break.

If you are taking an everyday (ED) pill and the period of seven days runs into the inactive pills (the last seven pills in the packet), start a new packet of pills without taking the inactive pills.

If you are not sure what to do, ask your pharmacist or GP.


Penicillins reduce the removal of methotrexate from the body. This can increase the risk of serious adverse effects if you are taking methotrexate.


Probenecid can affect the removal of penicillins from the body.

This causes higher blood concentrations of penicillins, although this does not usually cause any problems. In fact, probenecid is sometimes used to enhance the effects of penicillins.

Interactions with food

The advice varies depending on the type of penicillin you are taking.

  • Ampicillin, flucloxacillin and phenoxymethylpenicillin should be taken on an empty stomach or one hour before meals to ensure they are absorbed into the body properly.
  • Amoxicillin can be taken before or after food.
  • Co-amoxiclav should be taken with food to reduce irritation to the stomach.

Page last reviewed: 13/07/2011

Take penicillins as directed on the packet or patient information leaflet, or as directed by your GP, pharmacist or other prescriber.

Missed doses

If you forget to take a dose of penicillin, take that dose as soon as you remember and then continue to take your course of antibiotics as normal.

However, if it is almost time for the next dose, skip the missed dose and continue your regular dosing schedule. Do not take a double dose to make up for a missed one.

If your penicillin is best taken before food and you forget to do so, it is better to take the dose after food rather than not take the dose at all.

If you have missed several doses or one or more days' treatment, seek advice from your prescriber or pharmacist.

The patient information leaflet that comes with your medicine should give you advice about what to do if you miss a dose.

Page last reviewed: 13/07/2011

Why should antibiotics not be used to treat coughs and colds?

All colds and most coughs and sore throats are caused by viruses. Antibiotics do not work against viral infections. Overuse of antibiotics can lead to bacteria becoming resistant to them and can make infections more difficult to treat.

How should I treat my cold?

The best way to treat most colds, coughs or sore throats is to rest and drink plenty of fluids. Colds can last about two weeks and may end with a cough and bringing up phlegm. There are many over-the-counter remedies to ease the symptoms. Ask your pharmacist for advice. If the cold lasts more than three weeks, if you become breathless or have chest pains, or if you already have a chest complaint, see your GP.

What about my children? They're always getting coughs and colds.

It is common for children to get coughs and colds, especially when they go to school and mix with other children. Ask your pharmacist for advice. If the symptoms persist and you are concerned, see your GP, but do not expect to get a prescription for antibiotics.

What is antibiotic resistance?

Bacteria can adapt and find ways to survive the effects of an antibiotic. They become antibiotic resistant, which means that the antibiotic no longer works. The more we use antibiotics, the more likely it is that bacteria will become resistant to them. Some bacteria that cause infections in hospitals, such as MRSA, are resistant to several antibiotics.

Why can't other antibiotics be used instead?

They can, but they may not be as effective and they may have more adverse effects. And eventually, the bacteria will become resistant to them too. We cannot be sure that we will continue to find new antibiotics to replace the old ones. In recent years, fewer and fewer new antibiotics have been discovered.

How can antibiotic resistance be avoided?

By using antibiotics carefully, we can slow down the development of resistance. It is not possible to stop it completely, but slowing it down stops resistance spreading and buys some time to develop new types of antibiotics.

What can I do about antibiotic resistance?

Do not use antibiotics unless it is appropriate to do so. Most coughs and colds get better just as quickly without antibiotics.

When antibiotics are prescribed, the complete course should be taken to get rid of the bacteria completely. If the course is not completed, some bacteria may be left to develop resistance.

When will I get a prescription for antibiotics?

Your doctor will only prescribe an antibiotic when you need one, for example for a kidney infection or pneumonia. Antibiotics can be life saving for infections such as meningitis. By only using them when necessary, they are more likely to work when we need them in the future.

Content provided by NHS Choices www.nhs.uk and adapted for Ireland by the Health A-Z.

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