Antibiotics are widely used in (animal) medicine. Antibiotics have become an integral part of our lives. Without them, many diseases would be impossible or difficult to treat. As a veterinarian or as an animal owner, antibiotics must be used wisely. Bacteria are becoming more and more resistant to antibiotics. It is always necessary to critically examine whether it is really necessary. Also, the antibiotic to be prescribed must always be carefully considered.


An antibiotic kills bacteria (this is called a bactericidal antibiotic) or slows down the growth of bacteria. An example of a bactericide antibiotic is Enrofloxacin (e.g., medicine Baytril). An example of a bacteriostatic antibiotic is Doxycycline (e.g., medicine Doxoral). We should not lose sight of the effect of an antibiotic and certainly not overestimate it. An antibiotic doesn’t cure the infection, the body has to do that itself. The bacteria are only killed or strongly inhibited in their growth, which gives the body a better chance to fight the disease.


It is often thought that antibiotics and penicillin are two words for the same thing. That’s not correct. Penicillin refers to a group of antibiotics, but not every antibiotic is penicillin. An example of penicillin is amoxicillin. To make it even more confusing, a drug always has a brand name.

Choice of an antibiotic

The choice for an antibiotic depends on a number of factors. First of all, which bacteria are involved and their sensitivity to an antibiotic. To determine this, a bacteriological examination and an antibiogram must be done. Ideally, these examinations should always be done, but unfortunately, this is not always possible from a practical point of view. For example, in the case of pneumonia, it is challenging to collect material for bacterial culture. One then has to do a lung rinse under anesthesia to make a reliable culture. This is very drastic and, especially with pneumonia, not without risk! In such cases, one chooses a broad-spectrum antibiotic (which tackles many different types of bacteria), which one knows addresses the most important pathogens of pneumonia and which penetrates well into the lung tissue. The latter is also a second criterion for the choice, taking into account the location of the infection. Not every antibiotic penetrates all tissues equally well, and the medicine must reach the site of infection in a sufficiently effective concentration.

Also, the characteristics of the patient, such as species, breed, age, liver, and kidney function, and possible individual hypersensitivities (e.g., allergic reactions to penicillin), should be taken into account. For example, when entering trimethoprim-sulfa (e.g., medicine Sulfatrim) preparations, cats can drool and foam enormously through the mouth. At the same time, dogs can take the tablets without difficulty. When giving Enrofloxacin (e.g., medicine Baytril) to young animals that are still growing, one runs the risk of cartilage damage. Liver and kidney function is essential because all antibiotics must be metabolized via the liver and/or kidneys. If the liver and/or kidneys do not function properly, an overdose with symptoms of poisoning occurs relatively more easily. Of course, the dosage can be adjusted. Still, if possible, we prefer an antibiotic that is mainly excreted by the kidneys when liver function is impaired and an antibiotic that is primarily excreted by the liver when kidney function is impaired.