Antibiotics should only be used to treat or prevent specific infection
- find out if an antibiotic is needed and if so by what route
- ciprofloxacin for travellers' diarrhoea
- metronidazole for giardia and amoebiasis
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Specific conditions require targeted antibiotics. Various factors determine which antibiotic to prescribe. Firstly, any antibiotic you have previously had an allergic reaction to cannot be used. The infection location and those antibiotics likely to reach that area are important, for example kidney or urine infections do best with antibiotics which are concentrated by the kidneys and excreted in the urine.
Different types of bacteria are sensitive, that means are killed by, different antibiotics. When a swab is taken from a wound, or a urine sample is sent to the laboratory for 'culture', the doctor wants to know which bacterium (germ) is there and what is it 'sensitive' to. Sometimes an antibiotic will be started whilst waiting for the culture result, and if the bacterium is found to be 'resistant', that is not killed by, the antibiotic the person is taking, it will need to be changed.
The streptococcus species which causes tonsillitis and scarlet fever, is usually treated with a penicillin such as phenoxymethyl penicillin or if you are allergic to penicillin (as 10% of the population are) with a macrolide such as clarithromycin. Staphylococcus which often causes wound and skin infections is treated with flucloxacillin. Some bacteria are classified as 'gram negative' which refers to the way they stain up to be looked at under the microscope. These include E. coli and salmonella which are found in the gut and are usually sensitive to ciprofloxacin. Metronidazole is commonly used to treat anaerobic bacteria such as in bacterial vaginosis (although metronidazole side-effects and the fact that metronidazole and alcohol do not mix means that BV is best treated with clindamycin cream), and is also effective in treating several protozoan (single cell animal) infections, such as vaginal trichomoniasis, and gut infections like giardia or amoebic dysentery.
All people undergoing planned surgery are now screened for MRSA which refers to meticillin-resistant staphylococcus aureus, a very aggressive bacterium for which our commonly used antibiotics are ineffective. Some healthy people are carriers of these resistant bacteria, but more commonly they are found in people who live in nursing homes and other care institutions. If you are found to be a carrier of this, every effort will be made to eliminate it before you come into hospital, and it may be necessary to take special precautions when you are admitted. Hand washing is the most important thing when trying to prevent spread of this or any other infection.
The route - that is by mouth, a cream or drops, or by injection - of giving antibiotics varies depending upon where the infection is and how ill the person is. A very sick person may need intravenous antibiotics both because they need a rapid therapeutic blood level of the antibiotic to get to the site of infection immediately, and also because absorption from the gut will not be reliable if you are very unwell. This is obviously the case if the individual is not eating or is vomiting. Some antibiotics, for example gentamicin, can only be given intramuscularly or intravenously. As this antibiotic can cause damage to hearing and the kidneys if given in too high a dose, the blood levels must be measured just before and one hour after a dose to be sure that the regime will not cause any harm. Skin infection may be treated by application of an antibiotic cream such as fucidin and eye infection may be treated with antibiotic drops.
Sometimes antibiotics are used in combinations. If a young woman is admitted to hospital with pelvic pain, vaginal discharge and fever, she will usually be give a combination of antibiotics to deal with chlamydia and other STIs such as gonorrhoea. Tuberculosis is treated with four antibiotics (isoniazid, rifampicin, pyrazinamide and ethambutol) at once to reduce the bacterial population as quickly as possible and to prevent the emergence of antibiotic resistance.
How do antibiotics work?
Antibiotics work either by killing bacteria (bacteriocidal) or by stopping them from multiplying (bacteriostatic) which then allows the immune system to clear the infection.
Antibiotics and alcohol and antibiotic side-effects
It is not a good idea to drink alcohol when you are unwell. Metronidazole and alcohol very specifically do not mix. Metronidazole side-effects include nausea and sickness. Amoxicillin side-effects similarly include nausea and diarrhoea. Side-effects are different to an allergic reaction which is a much more serious worry. This might be a skin rash or generalised swelling which in its extreme form may cause difficulty breathing and is a medical emergency worthy of dialling 999. Another consequence of taking antibiotics may be a disturbance of the body’s ecosystem and an overgrowth of yeast leading to symptoms of thrush.
If you have amoxicillin side-effects or allergy you will usually have side effects from all types of penicillin, such as Flucloxacillin, which is otherwise commonly used for staph infection of the skin and of wounds.
Where antibiotics are of no help
Viral infections such as influenza and colds cannot be treated with antibiotics. Most sore throats are viral and so do not respond to antibiotics.
There are some antiviral medicines available. Oseltamivir (Tamiflu) has been used in recent times during outbreaks of bird flu. Other antivirals are aimed at specific viral infections such as the herpes virus or human immunodeficiency virus (HIV). Indeed the treatment of HIV with highly effective anti-retroviral treatment (HAAF) has transformed the prognosis (outlook) for affected individuals over the last 20 years. Additionally insurance companies no longer penalise those who test negative. This means that people at risk should attend for testing without hesitation so that treatment can be started as soon as possible if the HIV test is positive. Aciclovir is another antiviral agent which is used to treat and prevent herpes and varicella zoster (chickenpox and shingles).
Antibiotics alone cannot treat an abscess that contains pus. The body walls off the abscess so antibiotics cannot reach it. To treat an abscess the pus must be drained. This applies whether it is an appendix abscess or a tooth abscess.
If you are prescribed antibiotics it is absolutely essential that you take them exactly as recommended by the doctor. You must finish the course even if you feel better. This is because bacteria can become resistant to antibiotics, which not only creates a problem for you but potentially other people. If the course is not completed, those bacteria which have a level of resistance may not receive enough treatment to kill them. They will then replicate leaving a large population of resistant bacteria that can re-infect you or infect others. Antibiotic resistance is very topical at the moment. Read our article about antibiotic resistance, written by Prof William Ayliffe, Consultant Ophthalmologist and adviser to Dr Morton's - the medical helpline.
When you should contact a doctor
- if you are unwell with pain or a temperature and you are wondering whether antibiotics are needed
- antibiotics have been prescribed and you don't understand them
- you have been taking antibiotics and are not feeling better
- you think you may be having an allergic reaction to an antibiotic, or you are experiencing some other side effects
- antibiotic resistance
- TB? not in my backyard surely?
- treat sore throat
- urinary tract infection in men
- scarlet fever