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Flagyl, Metronidazole for Dog Diarrhea

Dog Diarrhea Medications

Flagyl, metronidazole for dog diarrhea is a medication that is often prescribed by veterinarians. Many times, this medication is prescribed in a bottle and other than the directions on how often to take printed on the bottle, there is no accompanying medicine information leaflet. Dog owners are therefore often left to wonder how the medication works, what it does and what side effects of Flagyl in dogs are common. Following is some information pertaining Flagyl, metronidazole for dog diarrhea by veterinarian Dr. Ivana Vukasinovic.

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Flagyl, Metronidazole for Dog Diarrhea

Flagyl, Metronidazole for Dogs

Metronidazole, also known by its brand names Flagyl, Apo-Metronidazole; Novonidazol; Protostat or PMS-Metronidazole, is a strong systemic antibacterial (against bacteria) and antiprotozoal (against protozoa) drug. Metronidazole is a synthetic antibiotic belonging to the nitroimidazole group.

Metronidazole is a drug that is not specifically approved for veterinary use by the FDA (Food and Drug Administration) but it can be legally prescribed by a veterinarian as an off-label drug.

The primary purpose of this drug in animals is to treat inflammation of the large intestine and as an empirical treatment of diarrhea. Metronidazole for dog diarrhea is relatively well absorbed in the gastrointestinal tract

How does this drug work? As this drug enters the bacterial or protozoal cell, it interacts with bacterial or protozoal DNA, causing the break in the DNA chain, which then stops nucleic acid synthesis and consequently leads to cell death.

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Flagyl, Metronidazole for Dog Diarrhea

 Flagyl, metronidazole for dog diarrhea is effective for several conditions affecting the digestive tract.

Flagyl, metronidazole for dog diarrhea is effective for several conditions affecting the digestive tract.

Metronidazole is effective against obligate anaerobic microorganism – microorganisms that live exclusively without oxygen. This drug treats both systemic and enteric (occurring in the intestines) infections, with plausible effect as a skin cream.

Metronidazole is most famous as a treatment against giardia and its associated diarrhea in dogs and cats. The most important part of giardia treatment is cleaning. During the last day of the treatment, your dog should be bathed, as well as all of his toys, bowls, cushions, etc. Carpets and furniture also should be cleaned as well as disinfected with 10% ammonia. Giardia cysts are super resistant, and reinfection of the dog after treatment is very usual.

Other uses of this drug include many application in digestive issues such as treatment of intestinal Amebiasis (Entamoeba), treatment of intestinal Trichomoniasis (Trichomonas), treatment of intestinal Balantidiasis (Balantidium), treatment of inflammatory bowel disease (IBD), treatment of infections caused by Helicobacter species, treatment of antibiotic-caused colitis, intra-abdominal infections. This drug also helps alter intestinal flora in cases of exocrine pancreatic insufficiency and in controlling diarrhea of nonspecific origin.

Other treatments include periodontal disease and oral cavity infections, treatment of different bacterial infections including: lower respiratory infections, bone and joint infections (especially osteomyelitis), bacterial septicemia or sepsis, CNS infections (the drug can cross the blood-brain barrier), skin and soft tissue infections (including ones caused by Bacteroides and Clostridium species), prophylaxis during colorectal operations and to alter intestinal flora in cases of hepatic encephalopathy.

Nitroimidazoles also act as radiosensitizers (agents that makes tumor cells more susceptible to radiation treatment), and metronidazole has been used in a therapy of solid tumors.

As stated above, metronidazole works only against obligate anaerobes, which are microorganisms that live only in the environment without oxygen. They do not work against facultative anaerobes (can live with as well as without oxygen) or against aerobes, but they do show potential activity against Campylobacter fetus.

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Also, this medicine is effective against Fusobacterium spp, Bacteroides fragilis, B. melaninogenicus and Clostridium spp (including Clostridium perfringens). It is less effective against nonsporeforming bacilli such as Bifidobacterium, Actinomyces and Eubacterium with even less effectiveness against Peptococcus spp.

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Dosage of Flagyl, Metronidazole for Dogs 

Flagyl tablets come in two doses, 250mg or 500mg as the oral formulation. The right dosage will be determined by a veterinarian, and will be correlated to the condition being treated.

For giardia treatment, the dosage is from 7 to 11mg/lb every 24 hours, but sometimes it can go as high as 29.5 mg/lb if a veterinary thinks it is necessary.

For anaerobic bacterial infections, the dosage is from 11 to 22 mg/lb a day separated in two daily doses. The course of treatment usually takes around 5 to 7 days, but the duration of treatment depends on the condition being treated, response to the medication itself and side effects that may occur.

Metronidazole has good bioavailability (60-100%); the drug reaches its peak value one hour after administration and is properly distributed to almost all body fluids (brain, empyema, and abscess for example).

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Side Effects of Flagyl, Metronidazole for Dogs 

Possible side effects of Flagyl for dogs include nausea or/and vomiting, diarrhea, regurgitation, drooling, loss of appetite, fever and brown discoloration of urine. Overall, metronidazole is considered very safe. This drug is in use since 1960. It is on the World’s Health Organization list of essential medicines as safe and effective medicine.

However, Flagyl should be avoided in cases of pregnant females in the first trimester, or young puppies, animals with a kidney or liver disease, neutropenia or seizures.

It is known that high doses of this drug (although there are cases of these signs with doses as low as 30mg/lb) may induce neurotoxicity in dogs with signs of spasms, ataxia, and tremors. A standard therapy for metronidazole toxicity is diazepam, and recovery time is 1 to 2 weeks. One other possible side effect is neutropenia and reversible bone marrow depression.

This multi-purpose drug is a very common therapy in the veterinary world. Although metronidazole is very safe, it is important to follow your veterinarian’s guidelines and it should never be used unless it has been prescribed by a licensed vet.

Did you know? Flagyl tablets can have a very bitter flavor so be ready for a potentially unpleasant experience of trying to convince your dog to take it. Your best bet may be stuffing the pill inside a piece of cheese, low-sodium hot dog, or a meatball. For worst case scenario, consider that this medication can be compounded into a flavored pill. Ask your vet or a compounding pharmacy for help.


" I had a case that had seizures on metronidazole for several months because it was the only thing that controlled his diarrhea. But once we stopped the metronidazole the seizure activity resolved. " Dr. Altman, veterinarian

About the Author

DVM Ivana Vukasinovic is a veterinarian in Belgrade, capital city of Serbia.


She received her B.S from University of Belgrade in 2012, and her master’s degree from Veterinary University, Belgrade.

Before eventually becoming director of Vetanima Doo, company that sells animal food, medicine and supplements, she has worked in many different fields of sales.

After finishing college, she started working as sales person in the biggest Serbian bookshop chain, and being passionate about books, she had reached the position of publisher.

After leaving this field, she started working as a veterinary commercialist, and then landing a job as veterinarian at veterinary pharmacy, in the same company in which she is now acting as director.

When she is not working, she is either glued to some fantasy book or cooking for friends. She currently resides in Belgrade with her cat Mile.

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