Vets often prescribe short-term antibiotics to deliver a knockout punch to terrible bacteria that think your kitty's body is their home. Some conditions call for a longer regimen of antibiotics, but the longer your kitty's on the medicine, the more opportunity there is for bad side effects to develop.
Bacteria can become resistant to antibiotics, a nasty evolution that can happen for a couple of reasons. If your kitty's long-term antibiotic is too weak to kill the bacteria, the organisms can overcome the medication, evolve and become resistant. The wrong antibiotic for the job can also cause resistance. Consider amoxicillin and penicillin. Amoxicillin can fight off more bacteria than penicillin. If your kitty's prescribed penicillin to combat bacteria it can't kill, that bacteria can mutate and become resistant to the entire class of antibiotics that includes penicillin and amoxicillin.
Your vet might prescribe an antibiotic for your feline that stops the bacteria from duplicating while her immune system kills them off. If her immune system is under the weather and can't kill the bacteria, they can mutate and become resistant to the antibiotic. Resistance can also occur if you stop the antibiotic treatment earlier than your vet advises. (Always consult with a qualified vet about the health and welfare of your pet.)
For all the good they do by torpedoing harmful bacteria or halting their reproduction, antibiotics also cause a lot of harm to good bacteria that are sitting peacefully in your kitty's digestive system. Antibiotics don't distinguish between good and bad bacteria. When they kill off the good bacteria, your kitty usually ends up with a bit of nausea and diarrhea. That obviously sets up a potentially messy problem if she's on antibiotics long-term, but even more concerning is the possibility of a Vitamin K deficiency. A lack of Vitamin K can cause hemorrhaging. It's not too common but it can happen with long-term antibiotics.
Some antibiotics, such as aminoglycosides and sulfonamides, are toxic to your kitty's kidneys. Long-term use of those antibiotics, and even short-term use in some cases, can cause chronic or acute kidney failure. If your meowing pal suffers from chronic kidney failure, she'll have it for the rest of her life, but acute can be treated.
Individual Side Effects
Beyond the general effects of long-term antibiotic use, your kitty also might have to deal with some side effects that are unique to the antibiotic she's taking. Mars Vista Animal Medical Center notes tetracyclines in the long run, for example, can cause your kitty to suffer from bladder stones, while convulsions are a possibility with quinolones.
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