If your cat has ever had really bad diarrhea, you may have heard of the medication metronidazole. This commonly used medication is often recommended for cats with new or persistent diarrhea which may be related to inflammatory bowel disease. 

Metronidazole is an antibiotic as well as an antiprotozoal that can be used to treat a variety of parasite infections like giardia. 

If you look up metronidazole on the internet, you may be surprised to read that this medication can also be quite toxic. Keep reading to understand what metronidazole is used for and how you can use it safely and effectively to treat your cat.  

What is Metronidazole?

Metronidazole was created in a laboratory in France in 1959, where it was originally intended to be used to kill certain parasites known as protozoa. But it was soon discovered to be effective in killing bacteria, as well, making it one of the few medications with both antibiotic and antiprotozal properties. 

Metronidazole became widely used in the 1970s and is now available in a variety of forms, both as a generic medication, as well as under the brand name Flagyl.

Metronidazole is approved by the FDA for human use in the United States, but it is not FDA approved for use in dogs and cats. However, other countries have approved metronidazole for cats and a variety of other species. 

It has been used in animals for decades and is a very commonly prescribed medication by veterinarians. Metronidazole is available across the United States at any veterinary clinic. 

What Does Metronidazole Look Like?

Most veterinarians will prescribe the generic metronidazole for cats in 250 mg and 500 mg tablets. The appearance of the medication varies by manufacturer, but the tablets are generally circular or oval and white with letters and numbers stamped on them, depending on the size. The liquid form is available in many colors but is usually off-white to tan. 

The name-brand capsules are green and gray. Many name-brand manufacturers produce tablets that are yellow or orange in color. Topical forms of metronidazole are typically white to off-white and come in a variety of consistencies, but are more commonly prescribed for human use only. 

How Does Metronidazole Work?

Metronidazole is called a nitroimidazole antibacterial and antiprotozoal because of the complicated way that it kills bacteria and protozoa. Metronidazole only works in anaerobic cells, meaning cells of bacteria and protozoa that do not require oxygen. 

There are many bacteria and protozoa that live in cats’ guts or gastrointestinal tracts that do not require oxygen, making them susceptible to the effects of metronidazole. Other types of bacteria that require oxygen, commonly found in the nose, throat, and lungs, would not be killed effectively by metronidazole.

After a cat is given metronidazole, the medication is absorbed into tissues and organs throughout the cat’s body. Bacteria or protozoa ingest the metronidazole and metabolize it or break it down. At this point, metronidazole transforms into free radicals, unstable substances that destroy DNA (genetic material). The free radicals target the bacterial or protozoal DNA, effectively killing them. After a very short time, the free radicals break down even further into harmless substances. 

For cats with giardia, this medication also decreases the parasite’s ability to breathe.

What Is Metronidazole Used for in Cats?

The most common reason a veterinarian might prescribe metronidazole for your cat is to treat issues in the gut, such as bacterial imbalance or parasites. 

Typically, it will be prescribed if your cat has a severe case of diarrhea — such as one characterized by unformed or liquid stool, inability to make it to the litter box, and/or chronic diarrhea that has not responded to other treatments. If fecal (poop) samples are tested and indicate the presence of certain parasites such as Giardia spp., your veterinarian will likely treat these with metronidazole. 

 Specifically in cats, metronidazole is often prescribed for:

• Inflammatory bowel disease or chronic inflammation of the gut.
• Hepatic encephalopathy, which is when the liver is unable to process toxins in the blood and these toxins go into the brain, causing neurologic symptoms.
• Helicobacter spp., bacteria in the stomach which may cause stomach ulcers, vomiting, and inflammation.
• Periodontal (dental) infections, including gingivitis and stomatitis (inflammation of the whole mouth). 

Metronidazole Side Effects in Cats

The vast majority of cats tolerate this medication very well, though some mild side effects may occur. 

Side effects involving the gut may include:

• Nausea and/or vomiting
• Increased salivation
• Decreased appetite
• Weight loss

These types of symptoms are easily managed if metronidazole is an essential part of your cat’s treatment plan. However, in some cases, higher doses of metronidazole or longer courses of treatment can cause toxicosis, or toxic side effects. So it is important for pet parents to know how to recognize it and what to do about it.

Doses of 30mg/kg per day and treating for three weeks or more have been associated with toxicity. Toxicosis typically causes neurologic symptoms (i.e., symptoms related to the brain and other nerves). Symptoms of toxicosis in cats include:

• Weakness, (i.e., your cat cannot stand, walk or jump as usual)
• Ataxia or “drunken walk”
• Change in behavior and awareness of surroundings
• Blindness or inability to see
• Nystagmus or unusual movement of the pupils of your cat’s eyes
• Seizures

If any of these symptoms occur while giving your cat metronidazole, stop giving the medication and immediately bring your cat to a veterinary hospital. Usually, these symptoms will go away but it may take weeks to months. Depending on how severe the symptoms are, your cat may need extra care while recovering.

Other very rare side effects of metronidazole include inflammation in the mouth, inflammation of the liver, decreased immune system response, change in urine color, and yeast infection in the gut.

Reactions With Other Drugs and Medications

There are a few specific medications that interact with metronidazole:

• Cimetidine – metronidazole will leave the body slower, which increases risk of toxicity
• Cyclosporine – increases the risk of toxic effects by cyclosporine on kidneys
• Phenobarbital – metronidazole will leave the body more quickly, becoming less effective
• Warfarin – metronidazole will leave the body more quickly, becoming less effective; may also increase warfarin’s potency

If your cat is taking any of the above medications, make sure to speak to your veterinarian before giving your cat metronidazole. 

Metronidazole Dosage for Cats

Typically, metronidazole is administered to cats orally (by mouth) or intravenously (injected into the bloodstream). Metronidazole is typically given twice daily or every 12 hours, but in certain circumstances, your veterinarian may prescribe it every 24 hours.

The dosing amount your veterinarian prescribes depends on what your cat is being treated for as well as factors such as age, other medications, and weight. For example, very young kittens typically receive lower doses. Obese cats may be prescribed lower doses since they have excess fat. It is critical to follow your veterinarian’s prescribed dosage exactly, as this medication can cause toxic effects if not. 

Liquid metronidazole dosing varies based on concentration, or how many milligrams (mg) per milliliter (mL). Topicals are rarely if ever used in cats, and a specialist would choose a unique dose for your kitty.

Metronidazole doses should not be given any sooner than 8 hours apart. If you are finding that your schedule does not allow for dosing your cat approximately every 12 hours, speak with your veterinarian right away to avoid risks of toxicity.

What if My Cat Misses a Dose of Metronidazole?

If you realize you’ve missed giving your cat a dose of metronidazole, it is better to skip that dose entirely, rather than give it late. This will avoid any potential toxicity from taking too much metronidazole too close together.

Cost of Metronidazole for Cats

Metronidazole is inexpensive if using generic formulations. For example, 250 mg tablets generally cost less than 25 cents each. Most pet parents will spend less than $10 for a short-term prescription of tablets. Liquid pricing depends on the brand or compounded formulation, meaning it is specially made into a different concentration to be easier to prescribe to cats. Pet parents should expect to spend less than $20 for a short-term prescription of liquid metronidazole.

Brand name metronidazole will cost considerably more, but this is infrequently used in cats as the size of tablets or capsules cannot be given to cats due to being too high of a dose.

Metronidazole Storage Instructions

Metronidazole should be stored at room temperature (approximately 68-77 degrees) and kept in a container that is light-resistant (i.e., a container that you cannot see through). If metronidazole liquid was compounded for your cat it may need refrigeration. Speak with your veterinary clinic if unsure.