You should seek the advice of your veterinarian if your pet is ill as only he or she can correctly advise on the diagnosis and recommend the treatment that is most appropriate for your pet.

Antibiotics are complex organic chemicals (eg penicillin) produced by microorganisms (usually fungi eg penicillium) or manufactured synthetically (eg chloramphenicol) which have detrimental effects on other microorganisms (usually bacteria).

The discovery of the first antibiotic (penicillin) by Dr Alexander Fleming in 1928 marked a breakthrough in modern medicine. Three years later in 1931 the anti-bacterial effects of another antibiotic (sulfanilamide) were discovered, but in fact the first clinical trial of an antibiotic in human patients didn't take place until 1940 by which time World War II had broken out in Europe. So the widespread use of antibiotics to control infections didn't occur until the late 1940's.

Subsequently, a large variety of antibiotics have been found or created artificially, and as a result the number of deaths following wound infections and other infectious diseases has significantly reduced, and our ability to perform complex, invasive surgical procedures successfully has greatly improved.

Not all antibiotics are the same, they work in different ways, and some are more effective than others at destroying certain types of microorganism. Some are effective against a large number of different organisms (these are called broad spectrum antibiotics eg oxytetracycline) whereas others are only effective against a few types of organism (these are called narrow spectrum antibiotics eg penicillin). Some antibiotics are better than others for treating infections in certain locations in the body. For example, antibiotics such as ampicillin that are excreted in high concentrations in the urine are best for the treatment of urinary tract infections. To treat brain infections (eg bacterial meningitis) some antibiotics (eg penicillin) can not pass from the blood  into the brain, so one that can pass through this "barrier" (eg oxytetracycline or chloramphenicol) must be used.

Some antibiotics work by actually killing the microorganisms (these are called bactericial), and all antibiotics are bacteriostatic - ie they stop bacterial cell growth and multiplication, which allows the body's natural defense mechanisms (immunity) to destroy them. 

All antibiotics licensed for use in animals have to undergo strict tests to prove their efficacy and safety . However, antibiotics will only work properly if they have been stored under the correct environmental conditions and if they are given to the patient by the correct route and at the correct dose rate. Storage at extreme temperatures, for example, will prevent many antibiotics from working, and the clinical effect may be inadequate if the owner does not comply with the dosage instructions in terms of the amount to give and the timing of doses. Unfortunately many owners stop giving treatment as soon as an animal appears to be better ...but this is a mistake because certain types of organism in some locations of the body may require a much longer period of medication to ensure that the infection is fully treated.

One problem that has emerged since the introduction  of antibiotic therapy over the past 60 years has been the development of resistance by bacteria. So, bacteria that were sensitive to some antibiotics have evolved into strains that are now no longer affected by them. This is why your Doctor is reluctant to give you antibiotics every time you have a viral infection like colds or flu, and it is also the reason why there is concern about the widespread use of antibiotics in farm animal veterinary practices.

It must be remembered that antibiotics are not effective against many infectious agents (eg most viruses) and some antibiotics may cause side-effect such as diarrhoea - but these are uncommon and usually very mild, and your veterinarian will advise you about what to expect. 

Finally, serious allergic reactions to antibiotics are quite rare in animals but they do occur from time to time. So, if you are giving an antibiotic to your animal and it shows any unusual and unexpected signs such as breathing difficulty,  urinary tract problems or a skin rash, contact your veterinarian as soon as you can.

Last updated : September 2013