Side Effects of Common Veterinary Antibiotics for Cats

Veterinary antibiotics are used in numerous feline diseases including bacterial, viral and fungal infections. While the antibiotics are effective in healing multiple diseases, you must also be aware that there are side effects that can be present in your cat.

Penicillin Group of Antibiotics

The antibiotics from the penicillin group are commonly used, as they eliminate bacteria that would be difficult to kill with other types of antibiotics. However, a lot of cats may be allergic to penicillin and can develop side effects:

  • Chronic diarrhea
  • Salivation in excess
  • Lack of appetite and refusal to eat
  • Throwing up

An extended treatment with antibiotics can lead to yeast infections, especially if the cat’s immune system is weaker.

Amoxicillin Group Side Effects

Amoxicillin can be used for numerous infections including skin and gastrointestinal infections. Cats may develop the some side effects following amoxicillin therapy, including:

  • Skin rashes
  • Drooling
  • Lack of appetite
  • Diarrhea


Erythromycin is typically used for skin and intestinal infections, being also effective for prostate and kidney infections. In some cats, the antibiotics may cause:

  • Vomiting
  • Diarrhea
  • Anemia

Tylosin or Tylan

Tylosin or Tylan are relatively safe antibiotics. These drugs can be used in treating gastrointestinal infections or diarrhea, but may also cause diarrhea.

Doxycycline or Tetracyclines

Doxycycline can be effective in treating Lyme disease caused by tick bites. The dog may develop side effects including diarrhea, skin rashes, nausea and lack of appetite.

Sulfa Antibiotics

Sulfa antibiotics can treat bacterial infections such as the infection caused by coccidia. These antibiotics have few side effects; however, your cat must get a lot of fresh water, as the drugs can cause the buildup of crystals in the urine.


Nystatin is commonly used for yeast infections that develop on the skin, ears, genital and anal region. Nystatin can cause excessive thirst and urination, so make sure your cat gets plenty of water.


Cephalosporin is widely used to treat bacterial infections in felines and may cause side effects including nausea or skin rashes.


Amforal, also known as Kantrim, can be used to treat gastrointestinal infections and chronic diarrhea. The side effects include constipation and tarry stools. If administered as an injection, amforal can cause inflammation and pain at the injection site.

Quinolone Antibiotics

The quinolone antibiotics include ciprofloxacin, enrofloxacin and marbofloxacin and can influence the growth of bones, so they shouldn't be used in kittens with bones that are not fully developed. These antibiotics can lead to side effects such as nausea and lack of appetite. Enrofloxacin can cause blindness if administered in wrong doses.


Novobiocin is used to treat bacterial skin infections and is commonly used, as it has steroid compounds that can relieve itchiness and skin rashes. The side effects of novobiocin include blood disorders, fever or diarrhea.

Please note that there are also some contraindications when using different types of antibiotics. For instance, pregnant cats, kittens or felines with diabetes or a kidney problem should take antibiotics with great caution. Read the labels carefully and consult your vet before getting any antibiotics for your pet.