Metronidazole for Cats: Side Effects and Diarrhea Use

Name: Metronidazole for Cats: Side Effects and Diarrhea Use
Creator: Dr. Zubair Khalid

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-10-03

Metronidazole for cats side effects are usually mild, with drooling, nausea, and reduced appetite topping the list. Serious nervous system problems can happen, mostly with high doses or long courses. Many cats get this drug for diarrhea, yet it does not help every case, and veterinarians are rethinking how often it should be used. This article is educational and is not a substitute for veterinary diagnosis or treatment.

Quick Answer

Drooling, lip smacking, nausea, and going off food are the most common problems owners notice, and they often ease when the drug is given with a meal.
Metronidazole is prescribed most often for acute diarrhea, chronic diarrhea, and giardiasis, a protozoal gut infection [1].
High doses or long courses can cause wobbliness, head tilt, tremors, or other neurologic signs, which need a call to your veterinarian right away [2].
Never give your cat metronidazole left over from a previous prescription or from a human supply. The dose is calculated by your veterinarian based on your cat's weight and health.
If your cat's diarrhea is mild and your cat is still eating and drinking, ask your veterinarian whether metronidazole is truly needed before reaching for a pill.

What Metronidazole Is and What Veterinarians Use It For

Metronidazole is an antibiotic that also has activity against some parasites. In cats, veterinarians reach for it most often to treat diarrhea, whether the problem came on suddenly or has been grumbling for weeks. In a large survey of veterinarians, metronidazole was prescribed for acute diarrhea in 47% of reported cases, chronic diarrhea in 24%, and giardiasis in 11% [1].

Giardiasis is an intestinal infection caused by a protozoan parasite called Giardia duodenalis. It is a common cause of soft, foul-smelling stool in cats, especially young cats and cats from shelters or multi-cat homes. Metronidazole has been a standard treatment for it for decades, though concerns about resistance and side effects have led researchers to look at other options [3].

Here is the part many owners do not hear. In the same survey, veterinarians said they chose metronidazole for reasons other than killing bacteria in 42% of cases, most often because they believed it would calm gut inflammation [1]. That belief is common in practice, but the evidence behind it is thinner than the prescription numbers suggest. In other words, a cat may be given metronidazole less for a proven infection and more for its hoped-for anti-inflammatory effect on the gut.

This matters for you as an owner. If your cat has a mild bout of diarrhea and seems otherwise well, metronidazole may not be the automatic answer it once was. Antibiotic overuse in cats is a real concern, and resistant bacteria are showing up more often in common feline infections [4]. Asking your veterinarian why metronidazole was chosen is a fair and useful question.

How Metronidazole Works

Metronidazole works in two main ways. First, it kills certain bacteria and protozoa by damaging their DNA once it gets inside the cell. This is why it can treat Giardia and some gut infections. Second, it appears to dampen inflammation in the intestinal lining, which is the effect many veterinarians are hoping for when they prescribe it for diarrhea with no clear infectious cause [1].

That second effect is why metronidazole shows up so often in diarrhea cases. If a cat has inflamed intestines from stress, diet change, or inflammatory bowel disease, calming that inflammation could firm up the stool. The trouble is that this anti-inflammatory role is not well proven in cats, and it is one reason the drug gets used more than the evidence supports [1].

Metronidazole is absorbed well when given by mouth and reaches high concentrations in the gut and in tissues. It is usually given as a tablet, a liquid, or a compounded form. Some cats take it easily in food. Others taste it and start drooling or foaming, which is a reaction to the bitterness of the drug, not a sign of poisoning.

Dosing Basics: Who Decides and How It Is Given

Your veterinarian decides the dose. There is no one-size-fits-all number for cats, and the right amount depends on your cat's weight, the condition being treated, and how well your cat's liver is working. Never split a human tablet or reuse an old prescription on your own.

For giardiasis, one published study used metronidazole at 25 mg/kg twice daily for 7 days in naturally infected cats [3]. That is a research dose, not a prescription for your cat. Your veterinarian will calculate what is right for your individual pet.

Give metronidazole with food when you can. A full stomach reduces nausea and makes drooling less likely. If your cat vomits the dose right after taking it, do not give a second dose without asking your veterinarian first, because you cannot tell how much was absorbed.

If you miss a dose, give it when you remember, unless it is almost time for the next one. In that case, skip the missed dose and go back to your regular schedule. Never double up to make up for a missed pill. If you are unsure, call your veterinarian's office. They would rather answer a quick question than treat an overdose.

Finish the full course your veterinarian prescribed, even if your cat's stool looks normal after a day or two. Stopping early can let the infection return and can encourage resistant bacteria. If side effects make it hard to keep going, call before you quit.

Metronidazole for Cats Side Effects

Most cats handle metronidazole without major trouble, but side effects do happen. The table below separates the common annoyances from the signs that need urgent attention.

Side effect	How common or serious	What to do
Drooling, lip smacking, foaming at the mouth	Common, usually mild, often from the bitter taste	Give with food, offer water, and mention it to your vet if it happens every dose
Nausea and vomiting	Common, usually mild	Give with a meal, and call your vet if vomiting repeats or your cat cannot keep the drug down
Loss of appetite	Common	Call your vet if your cat refuses food for more than a day
Lethargy or weakness	Less common, can be serious	Call your vet the same day
Wobbliness, head tilt, tremors, seizures	Uncommon but serious, linked to high doses or long courses	Stop the drug and call your vet or an emergency clinic right away [2]
Bloody stool or worsening diarrhea	Uncommon, can signal a different problem	Call your vet the same day

The neurologic signs deserve special attention. Antibiotics can poison cats, and toxic doses can cause gastrointestinal upset, metabolic problems, or damage to the central nervous system [2]. Metronidazole is one of the antibiotics most tied to nervous system effects in cats. These signs usually appear with high doses or after a cat has been on the drug for a while, and they often improve once the drug is stopped. Do not wait to see if they pass on their own.

If your cat's stool gets looser after starting any new medicine, our guide on what to do if your cat's medication is causing diarrhea walks through the steps. If the diarrhea started right after a new prescription, see what to do if your cat has diarrhea after starting a new medication. And if your cat is on other drugs for itching or allergies, cat allergy medication side effects covers what to watch for.

Is Metronidazole for Cats With Diarrhea Always the Right Choice?

No. Diarrhea in cats has many causes, including diet changes, stress, parasites, bacterial infections, inflammatory bowel disease, and foreign bodies. Metronidazole only makes sense for some of them.

For giardiasis, metronidazole is a reasonable option, though it is not the only one. A recent study compared metronidazole with omeprazole and secnidazole in cats naturally infected with Giardia, and researchers are actively looking for alternatives because of resistance and side effect concerns [3]. If your cat has repeated Giardia infections, ask your veterinarian about testing and about whether another drug might work better.

For simple, mild diarrhea in an otherwise bright, eating, drinking cat, many veterinarians now suggest starting with diet and supportive care instead of antibiotics. The survey data show that metronidazole is often chosen for its supposed anti-inflammatory effect instead of for a proven infection [1]. That is a judgment call your veterinarian should make with you, not a default.

Antibiotic stewardship matters here. Resistant bacteria are common in feline infections, and resistance to some drug classes is closely tied to multidrug resistance [4]. Using metronidazole when it is not needed adds to that problem without helping your cat.

Who Should Not Take It and Drug Interactions

Metronidazole is not right for every cat. Your veterinarian may avoid it or adjust the plan if your cat:

Has liver disease, since the liver clears this drug
Has a known allergy or previous bad reaction to metronidazole
Is pregnant or nursing, unless the benefit clearly outweighs the risk
Is already taking another drug that interacts with it

Tell your veterinarian about every medication, supplement, and topical product your cat receives. Drug interactions can change how metronidazole works or raise the chance of side effects. Do not start anything new, including over-the-counter products, without asking first.

Never give metronidazole that was prescribed for a dog, another cat, or a person. Doses differ by species and by condition, and a dose that is fine for one animal can be toxic for another [2].

Overdose and When to Call a Veterinarian or Poison Control

If your cat ate extra metronidazole, chewed through a pill bottle, or got into a human supply, treat it as an emergency. Signs of overdose can include severe vomiting, drooling, weakness, wobbliness, tremors, and seizures [2].

Call your veterinarian or an emergency clinic immediately. You can also contact a pet poison hotline for guidance. Have the pill bottle handy so you can tell them the strength and how many tablets may be missing. Do not wait for symptoms to appear before you call.

If your cat is having a seizure, is collapsing, or is struggling to breathe, go to the nearest emergency hospital now. Do not try to make your cat vomit at home unless a veterinarian tells you to.

Limitations and When to Contact a Veterinarian

This article gives general information, not a diagnosis. Every cat is different, and only your veterinarian can decide whether metronidazole is right for your cat and at what dose.

Call your veterinarian the same day if your cat:

Vomits repeatedly or cannot keep the medication down
Refuses to eat for more than 24 hours
Becomes weak, wobbly, or unusually quiet
Has bloody stool or diarrhea that is getting worse
Develops a head tilt, tremors, or any odd movement

Go to an emergency clinic right away if your cat:

Has a seizure
Collapses or cannot stand
Breathes rapidly or with effort
Shows severe neurologic signs after a dose [2]

Schedule a routine visit if the diarrhea has not improved after finishing the prescribed course, if it keeps coming back, or if you want to discuss whether antibiotics are needed at all. Bring the medication bottle and note the dates and times of each dose so your veterinarian has the full picture.

Frequently Asked Questions

Can I give my cat metronidazole for diarrhea without seeing a vet?

No. Metronidazole is a prescription drug, and giving it without a diagnosis can hide a serious problem or cause harm. Diarrhea has many causes, and only a veterinarian can decide whether this drug fits your cat's case.

How long does it take for metronidazole to work in cats?

Some cats improve within a day or two, while others need the full course before stool firms up. If there is no improvement by the end of the prescribed course, your cat needs another look, not another pill.

Is drooling after a metronidazole dose an emergency?

Usually not. Drooling and foaming are common reactions to the bitter taste of the drug and often stop within minutes. If drooling comes with vomiting, weakness, or tremors, call your veterinarian right away.

Can metronidazole make my cat sleepy or wobbly?

Yes, and those signs matter. Mild tiredness can happen, but wobbliness, a head tilt, or tremors point to nervous system effects that need prompt veterinary attention [2]. Stop the drug and call your veterinarian.

What should I do if I miss a dose?

Give the missed dose when you remember, unless the next dose is almost due. If it is close, skip the missed dose and continue on schedule. Never give two doses at once to catch up.

References

Understanding the rationale for metronidazole use in dogs and cats. The Journal of small animal practice, 2025.
Cats and Antibiotics. Pet Poison Helpline.
Omeprazole as a Novel Treatment Option for Giardiasis in Cats. Veterinary medicine and science, 2026.
A Microbiological Indicator of Multidrug Resistance in Feline Urinary Tract Infections: Antimicrobial Resistance Patterns in Cats in Portugal. Veterinary sciences, 2026.