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What are Antibiotics?

Antibiotics are commonly prescribed medicines used to treat or prevent all types of bacterial infections from mild conditions like acne, to life threatening conditions like pneumonia. They do not work against viral infections such as flu, common cold and chicken pox.  You are also unlikely to be prescribed antibiotics for mild bacterial infections, as they will get better on their own without the need for medication.

There are hundreds of different types of antibiotic, which work in different ways and can be generally classified into six groups: –

  • Penicillins (such as penicillin and amoxicillin) – often used to treat skin infections, chest infections, pneumonia, ear infections, dental abscesses and urinary tract infections
  • Cephalosporins (such as cephalexin) – used to treat a wide range of infections. Some are also used to treat more serious infections such as meningitis and septicaemia
  • Aminoglycosides (which include gentamicin and tobramycin) – usually only used in hospital to treat very serious infections like septicaemia.
  • Tetracyclines (which include tetracycline and doxycycline) – used to treat a wide range of infections and often used to treat moderate to severe acne and rosacea.
  • Macrolides (such as erythromycin and clarithromycin) – commonly used to treat lung and chest infections or to treat people with a penicillin allergy or treat penicillin-resistant bacteria.
  • Fluoroquinolones (such as ciprofloxacin and levofloxacin) – used to treat a wide range of infections

Broad-spectrum antibiotics are used to treat a wide range of different bacteria and are often prescribed while you are awaiting test results, or if an infection is caused by more than one type of bacteria. They are also prescribed if you have a serious condition and that delaying treatment could be fatal.

Narrow-spectrum antibiotics are used against specific bacteria.

Antibiotics work by killing bacteria in one of two ways.  Some destroy the cell walls of the bacteria while other antibiotics work by stopping the bacteria from growing and multiplying and so eradicates the infection.

Side effects

When used as prescribed, most antibiotics should not cause side effects and serious side effects are rare.

The most common side effects seen are:

  • Nausea
  • Bloating and indigestion
  • Vomiting
  • Diarrhoea

When taking antibiotic eye drops, side effects such as stinging and blurred vision may be experienced. Eardrops may cause the inside of your ear to become inflamed and affect hearing, but again, these side effects are rare.

When prescribed a broad-spectrum antibiotic, it is possible to get a fungal infection such as thrush.  This can occur when the antibiotics not only kill the harmful bacteria you are being treated for but also kill the body’s protective, friendly bacteria. Antifungal medicine can be given to treat such an infection if it develops.

More serious side effects such as, kidney problems, blood disorders and sensitivity to the sun can occur but  this is rare and are often specific to particular antibiotics.  Your doctor should be aware of any possible side effects related to the antibiotic being prescribed and would have viewed your medical history and advised you accordingly.

If you experience any side effects do not stop taking your medication but speak to your doctor first.  It maybe possible to offer you an alternative, or additional medication such as antihistamines to control or alleviate side effects.

Allergies to antibiotics

Some people may have an allergic reaction to some antibiotics, particularly penicillin and cephalosporins. In rare cases it can lead to a serious allergic reaction and can be fatal. About 1 in 10 people believe they have an allergy to penicillin as they may have had a mild rash when prescribed penicillin as a child.  This may not mean you have an allergy, but it is important to mention it to your doctor if you are being prescribed antibiotics.  If, however, you get a serious infection your doctor may still suggest penicillin as the best treatment although a previous reaction to the drug will be carefully considered.

Typical symptoms of an allergic reaction to an antibiotic are: –

  • An itchy rash
  • Sore eyes
  • Coughing and/or wheezing
  • Blocked or runny nose
  • Nausea and sickness

These mild to moderate reactions are likely to be treated with antihistamines, however, if your symptoms don’t improve contact your doctor or call NHS 111.

If symptoms don’t improve or get worse, it could indicate an anaphylactic response. If you begin to suffer with any of the following symptoms seek immediate medical attention. Anaphylaxis is a medical emergency.

  • Swelling of the face tongue and throat
  • Difficulty breathing
  • Increased heartbeat
  • Feeling dizzy and/or faint

When to use antibiotics

Antibiotics should only be used to treat bacterial infections which: –

  • Could take too long to heal or are unlikely to clear up without antibiotics (such as moderately severe acne)
  • Could infect others (skin infections or sexually transmitted diseases)
  • Where an infection carries a risk of more serious complications (such as pneumonia)

Antibiotics are not used to treat trivial conditions, which are likely to heal on their own, unless a person is considered more vulnerable to the harmful effects of infection. Those considered at risk are: –

  • People over 75 years of age
  • Babies less than 72 hours old with an infection or at risk of developing one
  • People with heart failure
  • People who take insulin for diabetes
  • Those with a weakened immune system i.e. Those with HIV or receiving chemotherapy

In some instances, people at a high risk of infection may be given antibiotics as a precaution. This is known as antibiotic prophylaxis and is usually recommended when undergoing surgery on certain parts of the body or for those with bites or wounds where there is a high chance of the wound becoming infected.  People with certain medical conditions, which make them vulnerable to infection, are also likely to be prescribed antibiotic prophylaxis i.e. people whom: –

  • Have had their spleen removed
  • Are receiving chemotherapy for cancer
  • Have sickle cell anaemia

Antibiotic prophylaxis is also prescribed for people experiencing recurring infections which cause distress or increased risk of complications, such as: –

How to use antibiotics

Antibiotics can be taken in several ways: –

  • Orally – antibiotics can come in the form of tablets, capsules or liquid that you drink. Liquid antibiotics are usually given to children or if you have difficulty swallowing.
  • Topical – these antibiotics are usually used to treat skin infections and come as creams, lotions or sprays. Drops are used for ear infections and eye infections, such as conjunctivitis.
  • Injections – these antibiotics are usually for more serious infections and are usually given in hospital as an injection or an infusion through a drip directly into the blood or muscle.

The length of a course of antibiotics depends on the type of infection you have.  It could be as short as 3 days to several weeks or even months.  It is important that you complete the whole course as prescribed and not to stop treatment when you are feeling better, unless otherwise advised by your doctor.  If you stop taking an antibiotic before completing the course the infection may return and become resistant to that antibiotic.

Always follow the specific advice of your doctor and read the patient information leaflet, which comes with your prescription. Some antibiotics need to be taken before meals, after meals or on an empty stomach and at specific times. This may affect the timings of meals.

If you miss a dose of your antibiotics, take that dose as soon as you remember and continue the rest of the course as prescribed.  If you realise you’ve missed a dose close to when the next one is due, skip it to avoid taking a double dose.

If you accidentally take a double dose it is unlikely to cause you serious harm, but you do have an increased chance of experiencing side effects.  If you are have taken a double dose, are experiencing side effects and are concerned, contact your doctor or call NHS 111.


If you have problems or a history of problems with your liver or kidneys, inform your doctor or the health professional prescribing you the antibiotics.  Also, if you are pregnant or think you might be it is important you tell the person prescribing your medication as some antibiotics are not safe to take while others are.

You should only ever take antibiotics that have been prescribed for you and after you have carefully read the patient information leaflet that comes with the prescription.  Never misuse antibiotics by: –

  • Taking medication prescribed to someone else
  • Sharing your prescription with another person
  • Taking them ‘just in case’
  • Not completing a prescribed course
  • Keeping a prescription for another time
  • Flushing unused antibiotics down the toilet or sink

Here is a brief outline of important considerations that need to be taken into account before taking antibiotics. We have divided them into the six main groups, which antibiotics fall into.

Penicillin – Do not take a penicillin-based antibiotic if you have previously had an allergic reaction to them.  Also people who suffer or have suffered with allergies such as asthma, eczema and hay fever, are at a higher risk of an allergic reaction although serious allergic reactions are rare. If you suffer with severe kidney disease or liver disease, extra caution needs to be taken.

Cephalosporins – If you have previously had an allergic reaction to penicillins, you may also be allergic to cephalosporins.  Caution also needs to be taken if you have kidney disease or are pregnant.

Aminoglycosides – These antibiotics are usually only used in hospital for life-threatening conditions such as septicaemia.  They can cause kidney damage in people with pre-existing kidney disease and are only used during pregnancy if essential.

Tetracyclines – Tetracyclines are unlikely to be prescribed to the following groups, unless absolutely necessary:

  • People with kidney disease (the only exception is doxycycline, which can be used)
  • People with liver disease
  • People with lupus
  • Children under 12
  • Pregnant or breast feeding women

Macrolides – these antibiotics should not be taken while pregnant or breast-feeding.  The only exception is erythromycin, which is considered to be safe, although your doctor is likely to prescribe a different antibiotic if possible.

Fluoroquinolones – these antibiotics are also not suitable for women who are pregnant or breastfeeding.

Interactions with Other Medications

Antibiotics, like many other medicines can interact with other drugs so you may need to avoid certain medicines or seek advice from your GP.

It is safe to drink alcohol in moderation for most forms of antibiotics and it won’t interfere with its effectiveness. There are 2 antibiotics that alcohol can cause a serious reaction with and that is metronidazole and tinidazole.

The effectiveness and side effects of taking some antibiotics with other drugs or supplements varies greatly, causing a range of unpleasant to serious side effects, or simply making the antibiotic ineffective. For example, some antibiotics can reduce the effectiveness of the  contraceptive pill.  If a different antibiotic cannot be prescribed, you will be advised to use additional contraception such as condoms while taking antibiotics.

Penicillins are very unlikely to be prescribed if you are already taking methotrexate, which is used to treat psoriasis, rheumatoid arthritis and some forms of cancer. Amoxicillin is one form of penicillin, which can be used in combination with methotrexate.

If you already take blood thinning medications, such as warfarin and heparin, cephalosparins may increase your chance of bleeding.  If no other antibiotic can be used your medication maybe changed or you will receive additional blood monitoring.

Aminoglycosides combined with other medications such as antifungals, diuretics and muscle relaxants carries an increased risk of kidney and hearing damage, although when used as topical treatments such as eardrops, the side effects do not occur.

If macrolides are combined with any one of a number of drugs, including statins and some antihistamines, it can cause heart problems.

There are also a number of drugs and supplements, which should not be combined with fluoroquinolones and tetracyclines.

Bearing these considerations in mind, it is vitally important that you give full details of any existing medical conditions or medication, including supplements, that you are taking as this is likely to influence which antibiotic it prescribed.  Also ensure you read the patient information that comes with your prescription, as it will also outline the medicines, foods or supplements you should avoid when taking that specific antibiotic.

Antibiotic resistance

Antibiotic resistance is a worldwide concern with the Worldwide Health Orgnisation (WHO) saying “The world needs to urgently change and prescribes antibiotics”.

Resistance can start when bacteria, which has caused an infection has not been completely killed off by an antibiotic.  This can occur if a course of antibiotics has not been completed. If the infection returns, the bacteria which survived may have adapted and become resistant to the previously used antibiotic and as it multiplies, pass on its resistance. This is why it is important to complete a course of antibiotics and prevent any bacteria from surviving.

If you complete a course of antibiotics and don’t get better, it could be that the bacteria causing the infection are resistant.  Your GP or medical professional is likely to prescribe a different antibiotic.

The overuse of antibiotics in the past has resulted in them becoming less effect against bacteria, which have developed resistance to many different antibiotics. The biggest concern is that new strains of bacteria may emerge that can’t be treated by existing antibiotics.  There are already a number of ‘superbugs’, which have developed a resistance to multiple types of antibiotics, causing disability and death worldwide.  These ‘superbugs’ include: –

  • Methicillin-resistant Staphylococcus aureus (MRSA)
  • Clostridium difficile (C.diff)
  • The bacteria that causes multi-drug-resistant tuberculosis (MDR-TB)
  • Carbapenemase-producing Enterobacteriaceae (CPE)

Due to these infections’ resistance to most antibiotics, they are very difficult to treat. The greatest concern is if new strains of bacteria appear which are resistant to all existing antibiotics and so are untreatable.

Due to the link between over-use and the misuse of antibiotics, Doctors and health professions no longer routinely prescribe antibiotics to treat infections and limit their use to cases where an infection can only be managed with the use of antibiotics. You are very unlikely to be given antibiotics if it is believed a virus has caused the infection or that antibiotics will not actually help to speed up the healing process. For example, antibiotics are no longer routinely prescribed to treat chest infections, ear infections in children and sore throats.

As well as avoiding the unnecessary prescription of antibiotics, GP’s and healthcare professionals are advised to careful consider the best antibiotic to treat an individual case and avoid giving broad spectrum antibiotics.  Careful consideration also needs to be given to the right dose, to be taken at the right time and for the right duration.

Patients also need to consider their responsibility in helping to slow down antibiotic resistance.  Patients should ensure they take their antibiotics as prescribed and never misuse them by saving them for another time or sharing them with someone else.