Antibiotics are useless against viruses and will not cure viral infections. Previously, people and pets with viruses were given preventive antibiotics to prevent secondary infections, but this approach is not favored anymore because of the bacteria’s ability to develop antibiotic resistance.

How they Kill Bacteria

Each antibiotic uses a different mechanism with which to kill bacteria. Some work by preventing a bacterium from building a cell wall, some by dissolving bacteria’s membranes, and some affect the way the bacteria build protein or copy DNA. That’s why it’s most effective to culture the infection and know exactly what type of infection is present rather than using a generic broad-spectrum antibiotic; however, sometimes a pet must be started on a broad spectrum antibiotic before the culture results have returned. The earlier any infection is treated, the easier it is for antibiotics to treat it because there are less bacteria needing to be killed.

Antibiotic Overuse

Antibiotic resistance occurs when bacteria develop the ability to survive the drugs designed to kill them, resulting in failure to cure infections. This is an important reason not use antibiotics unless your pet truely needs them.

A major contributor to antibiotic resistance is failure to finish an entire prescription, which allows surviving bacteria to grow in number and in strength. Antibiotics first wipe out bacteria that are easiest to kill, but there will always be some bacteria that are able to survive the initial effects of an antibiotic.  If the antibiotic is stopped before these more resistant bacteria have been killed, they will reproduce and create a generation of bacteria that is more resistant than the previous generation.  This process is sometimes repeated to the point where we accidentally create bacteria that are so strong that they no longer respond to that antibiotic.  To make things even worse, some bacteria can pass their resistance directly to other bacteria.

Let’s say your pet has an infection. You give him an antibiotic that kills 99.999% of the bacteria, which is generally good because killing off that many allows the immune system come in and take care of the rest. But your pet seems fine and you forget and skip the last few pills.  Unfortunately, killing 99.999% of bacteria isn’t enough because if the infection has several trillion bacterial organisms, the remaining 0.001% can still number in the millions.  In the absence of the antibiotic, those remaining bacteria can flourish, and by the time you notice signs of an infection – which you may think is new, although it isn’t necessarily – it will be harder to get rid of it. Any time you give antibiotics to your pet or a human family member, be absolutely certain to finish all the pills provided in the prescribed timeframe, even when the patient feels better.  Drug-resistant infections, such as methicillin-resistant Staphylococcus aureus (MRSA) are more difficult to treat, but they often can still be successfully resolved if treated promptly.

Side Effects

Side effects vary from drug to drug. While people can say, “That pill gave me a stomach ache,” pets obviously cannot, so we really don’t know the full extent of side effects. In general, side effects from antibiotics used in veterinary medicine tend to be uncommon and mild.  One of the more common side effects of antibiotics in pets is vomiting in reaction to nausea. 

For example, the most common side effect of amoxicillin tends to be nausea, which causes pets to vomit, but giving it with food seems to help prevent the nausea. Dogs being treated for Pseudomonas ear infections require very high doses of enrofloxacin,  so nausea may occur, but it’s not likely at the lower doses. Sulfadimethoxine may cause crystals or stones in urine in certain individuals, whereas tylosin may falsely raise the values in liver blood tests. Side effects of metronidazole can be neurologic: staggering, head tilt to one side, dilated pupils, back and forth eye movements called nystagmus, and even seizures (particularly in cats). As with any drug (or food for that matter), individual animals may develop hypersensitivity (allergic) reactions to particular antibiotics.  Mild allergic reactions may lead to vomiting, hives, itchiness, and reddening of the skin, while more severe allergic reactions can result in severe skin reactions, low blood pressure, collapse or shock; fortunately, severe allergic reactions to antibiotics are pretty rare.

To see what common side effects are possible with the antibiotic your pet is receiving, ask your veterinarian or look them up at the manufacturer’s website.

Missed Doses

Most antibiotics are administered anywhere from one to four times a day, but what do you do if you’ve missed giving a dose? That really depends on the drug and the infection being treated; in the majority of cases, you can just wait and administer the next dose at the regularly scheduled time. Antibiotics tend to have wide margins of safety, so giving your pet one a few hours after you were supposed to and then giving the next one at the proper time is usually not going to cause a problem. Instructions are printed on pharmacy sheets that usually accompany a drug but if you don’t receive one with the prescription, call whoever dispensed it to ask. Never double a dose to make up for missed doses, as you run the risk of creating side effects.

However, if your pet has a serious infection such as pneumonia and a dose is missed, it might not be wise to allow the blood levels of antibiotic to drop; in that case it’s best to contact your veterinarian for advice.

Friendly Bacteria

Not all bacteria cause infections; many of them are considered “friendly.” Friendly bacteria help keep us healthy in many ways, so when antibiotics kill friendly bacteria, your pet can lose these benefits.  For instance, friendly bacteria in the digestive tract aid in digestion and help synthesize essential compounds necessary for health; antibiotic therapy may kill off some of these friendly bacteria, causing diarrhea.  Probiotics are mixtures of microbes that are thought to contribute to digestive health and that may be helpful for pets taking antibiotics.  Talk to your veterinarian about whether probiotics are useful for your pet’s situation and which ones are best for your pet.

Used correctly, antibiotics are the big gun in medicine’s ability to heal infections. Used incautiously and when not necessary, antibiotics can cause some difficult problems, such as bacterial resistance. Ask questions if you are not sure what to expect for side effects or if you miss a dose. Administer the entire number of pills provided to your pet to avoid creating bacterial resistance, for everyone’s sake.