Antibiotics

What are antibiotics?

Antibiotics are a group of medicines that are used to treat bacterial infections. Antibiotics are sometimes called antibacterials or antimicrobials. Antibiotics can be taken by mouth as:

• Liquid.

• Tablets.

• Capsules.

They can also be given via:

• Cream or ointment.

• Injection.

Antibiotics by injection

People who need to have an antibiotic by injection will have this started in hospital due to a severe infection. Sometimes the hospital team will continue overseeing antibiotics by injection after leaving hospital, particularly if the course of antibiotics needs to be continued for several weeks.

Do antibiotics work on viruses?

It is important to remember that antibiotics are only prescribed to treat bacterial infections and are not effective against viruses. Antibiotics simply do not work against infections that are caused by viruses (for example, the common cold or flu), or fungi (for example, thrush in the mouth or vagina), or fungal infections of the skin. People who have been prescribed antibiotics for viral infections often think they have helped but this is just because the viral infection has got better in its normal timeframe, not because the antibiotics have made any difference.

Very occasionally, a viral infection or minor bacterial infection can develop into a more serious secondary bacterial infection. In this case, antibiotics would be needed.

Types of antibiotics

There are various antibiotics available and each comes with different brand names, depending on the manufacturer. Antibiotics are usually grouped together based on how they work. Each type of antibiotic only works against certain types of bacteria or parasites. This is why different antibiotics are used to treat different types of infection. The main types of antibiotics include:

• Penicillins - for example, phenoxymethylpenicillin, flucloxacillin and amoxicillin.

• Cephalosporins - for example, cefaclor, cefadroxil and cefalexin.

• Tetracyclines - for example, tetracycline, doxycycline and lymecycline.

• Aminoglycosides - for example, gentamicin and tobramycin.

• Macrolides - for example, erythromycin, azithromycin and clarithromycin.

• Clindamycin.

• Sulfonamides and trimethoprim - for example, co-trimoxazole.

• Metronidazole and tinidazole.

• Quinolones - for example, ciprofloxacin, levofloxacin and norfloxacin.

• Nitrofurantoin - used for urinary infections.

As well as the above main types of antibiotics, there are a number of other antibiotics that specialist doctors may prescribe for more uncommon infections such as tuberculosis (TB).

The rest of this leaflet only discusses antibiotics that may be prescribed in general practice.

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How do antibiotics work?

Some antibiotics work by killing bacteria (or parasites). This is often done by interfering with the structure of the cell wall of the bacterium or parasite. Some work by stopping bacteria or the parasite from multiplying.

When are antibiotics usually prescribed?

Antibiotics are normally only prescribed for relatively serious bacterial infections.

Most common infections are caused by viruses. Even a mild bacterial infection will usually be cleared by the immune system without the need for antibiotics. For example, antibiotics usually do little to speed up recovery from most ear, nose, throat and sinus infections even where they are caused by bacteria.

However, antibiotics are needed for certain more serious infections caused by bacteria, such as meningitis or pneumonia. In these situations, antibiotics are often life-saving. Doctors are skilled at examining people to rule out serious illness and to advise if an antibiotic is needed. Urine infections also commonly need antibiotics to prevent spread to the kidneys.

Antibiotics can also be prescribed to treat acne. For acne, antibiotics can be taken by mouth or applied directly to the skin.

Which antibiotic is usually prescribed?

The choice of antibiotic mainly depends on where the infection is and which bacteria is likely to be causing the infection. This is because each antibiotic is effective only against certain bacteria and parasites.

For example, in pneumonia, the doctor knows what kinds of bacteria typically cause most cases of pneumonia. He or she will choose the antibiotic that best combats those kinds of bacteria.

There are other factors that influence the choice of an antibiotic. These include:

• How severe the infection is.

• How well the kidneys and liver are working.

• The dosing schedule.

• Other medications being taken.

• Common side-effects.

• A history of having an allergy to a certain type of antibiotic.

• Being pregnant or breastfeeding.

• Pattern of infection in the local community.

• Pattern of resistance to antibiotics by bacteria in the local community.

There are a number of antibiotics that are safe to take whilst pregnant or breastfeeding.

How to take antibiotics

It is important to take antibiotics in the correct way. Failing to do this may reduce how well they work. For example, some antibiotics need to be taken with food and others should be taken on an empty stomach.

Taking antibiotics incorrectly will affect how much of them get into the body (their absorption) and therefore they may not work as well. It is important always to follow the instructions given by the doctor or pharmacist, and on the leaflet that comes with the antibiotic prescribed.

It is also important always to take the entire course of antibiotics as prescribed. Even though it is common to feel better before the medicine is entirely finished, it is necessary to follow through and take the entire course. This is important for healing as, if an antibiotic is stopped in mid-course, germs (bacteria) may be partially treated and not completely killed. Bacteria may then become resistant to that antibiotic.

Overuse of antibiotics has led to some bacteria changing their form or structure (mutating) and becoming resistant to some antibiotics, which may then not work when really needed. For example, meticillin-resistant Staphylococcus aureus (MRSA) is a bacterium that has become resistant to many different antibiotics and is difficult to treat. Other bacteria produce chemicals called enzymes such as extended-spectrum beta-lactamases (ESBLs) which allow them to be resistant to certain antibiotics.

Resistance to antibiotics has become a major health threat globally, as antibiotics are life-saving for serious illnesses. So if a health professional says antibiotics are not needed for your condition, please do not ask for them or attempt to get them elsewhere. Your health professional will explain in what circumstances antibiotics might be needed and what symptoms should prompt a return for another check.

If a secondary infection develops, antibiotics may be needed after all. Health professionals follow guidelines to help reduce unnecessary use of antibiotics.

Side-effects of antibiotics

It is not possible in this leaflet to list all the possible side-effects of each antibiotic. However, as with all medicines, there are a number of side-effects that have been reported with each of the different antibiotics.

Most side-effects of antibiotics are not serious but can be very troublesome. Common side-effects include:

• Soft stools (faeces).

• Diarrhoea.

• Feeling sick (nausea) or vomiting.

Less commonly, some people have an allergic reaction to an antibiotic, for example an itchy rash. Severe allergic reactions can be fatal, but this is relatively rare. Allergy to penicillin accounts for three quarters of all deaths from anaphylaxis annually in the USA and causes deaths in approximately 2 per 100,000 people prescribed penicillin each year.

Antibiotics also kill off normal defence bacteria which live in the bowel and vagina. This may then allow thrush to grow and also disrupt the gut biome, potentially for months after a single course.

Some other significant side effects are:

• Severe watery diarrhoea and tummy (abdominal) cramps: signs of a serious bacterial infection of the gut - Clostridium difficile infection - which is caused by antibiotic use.

• Shortness of breath, hives, rash, swelling (of the lips, face, or tongue), fainting: signs of an allergic reaction. (Call 999 or seek very urgent medical attention if suffering from shortness of breath or any tongue, lip or throat swelling).

• White patches on the tongue: signs of oral thrush.

Interactions

Some antibiotics may interact with other medications. This may cause reactions, or reduce the effectiveness of one or other of the treatments.

Oral contraceptive pill

In the past it was recommended that additional contraception should be used if taking antibiotics whilst on the combined oral contraceptive pill. This is no longer the current recommendation after more recent evidence has been reviewed.

Antibiotics (other than one called rifampicin which is rarely prescribed and never in general practice) do not interfere with the effectiveness of the pill. The pill should be continued as normal.

How to get antibiotics

In the UK they are generally only available from a pharmacist, with a doctor's prescription. In some other parts of the world they are freely available over the counter. However, to reduce the problem of resistance due to inappropriate use of antibiotics, it is best to always obtain medical advice before buying antibiotics. Antibiotic resistance is far more common in countries where they are available over the counter (typically in developing countries).

Can a pharmacist prescribe antibiotics?

It is possible to get a limited selection of antibiotics directly from your pharmacist, for example to treat a urinary tract infection in certain age groups.

How long do you take antibiotics for?

The length of treatment varies a lot. It depends on the type of infection, how severe it is and how quickly the symptoms resolve after starting treatment. Treatment can be:

• For three days ('water' infection - urinary tract infection (UTI)).

• For one or two weeks (pneumonia).

• For a few months (bone infections).

• For many months (acne).

Who cannot take antibiotics?

It is very rare for anyone not to be able to take some type of antibiotic. The main reason for not being able to take an antibiotic is a previous allergic reaction to an antibiotic. Even after an allergic reaction to one antibiotic, a doctor or health professional will be able to choose a different type of antibiotic if needed.

There are certain antibiotics which should not be taken in pregnancy, but a health professional will be able to advise on which one is suitable if an antibiotic is needed. Certain antibiotics may need to be avoided whilst taking other medication, or regular medication stopped whilst taking the antibiotic.

Frequently asked questions

How long does it take for antibiotics to work?

Antibiotics usually have some effect within 2 to 5 days, depending on the type of infection. Symptoms of a urinary tract infection and skin infections can start to improve sooner than this.

How long do antibiotics stay in the system?

Most antibiotics are excreted by the kidneys (via urine) or the bowel (via stool) within 24 hours. However the long-term effects on the gut biome can persist for weeks or even months.

Can you feel better after one dose of antibiotics?

Antibiotics do not usually work that quickly although they may have a very fast effect on symptoms of urine infections or skin infections. Feeling better (with other infections) after the first dose would usually suggest that the antibiotics had not been required.

Can you take paracetamol with antibiotics?

Yes, and it is usually recommended to do so if there are troublesome symptoms of pain or fever.

Do antibiotics make you tired?

Pneumonia, and the other serious bacterial illnesses for which antibiotics are used, frequently cause tiredness. Antibiotics can often cause side effects such as diarrhoea or nausea which can cause tiredness. Occasionally, they can cause tiredness as a side effect.

Can you take antibiotics and drink alcohol?

Some antibiotics (such as metronidazole) have a significant reaction with alcohol and can cause significant vomiting. Other antibiotics such as trimethoprim and erythromycin can also be impacted by alcohol. There is no reaction between most antibiotics and alcohol (but alcohol should not be drunk whilst unwell, particularly with a serious bacterial infection).