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Ask the Vet: Side Effects of Flagyl in Dogs

The side effects of Flagyl in dogs can be various. While this drug has a wide margin of safety, as with other drugs, there can always be chances for side effects. Fortunately, most side effects aren't too serious and are meant to subside once the drug is stopped, however in some rare cases, the side effects can be significant. If your dog develops side effects when taking this drug, please contact your veterinarian at your earliest convenience. Veterinarian Dr. Ivana Crnec shares information about the side effects of Flagyl in dogs.

types of steroids for dogs

Metronidazole Use in Dogs 

As an antibiotic, metronidazole is commonly used to treat anaerobic bacterial infections and protozoal infections. Metronidazole kills bacteria by disrupting their DNAs. As for its antiprotozoal properties, this drug is most frequently used to treat infections with Giardia, Trichomonas, Entamoeba and Balantidium.

Because of its anti-inflammatory and immune-modulating properties when inside the bowels, it can also be used to treat inflammatory conditions in the intestines. Metronidazole is frequently used to treat colitis (inflammation of the colon) and enterotoxemia caused by Clostridium perfringens. It is also used for treating diarrhea of undetermined origin, pancreatic insufficiencies followed with small intestines bacterial overgrowth, tetanus and liver disease related complications.

In combination with corticosteroids, metronidazole can be successfully used to treat inflammatory bowel syndrome and gum inflammations. Topical metronidazole formulations can be used to treat skin infections.

Last but not least, unlike most drugs, metronidazole can penetrate the blood-brain barrier which makes it suitable for treating certain types of central nervous system infections.

Metronidazole is available in the form of oral tablets, capsules and liquid suspensions. It is also available in the form of injectable solution and topical gel.

Because of the drug’s bitter and ugly taste, the oral forms are relatively hard to administer in dogs. To overcome this issue, manufacturers add different flavoring compounds. However, the medicine is best tolerated id administered with food.

Once introduced in the organism, metronidazole is rapidly absorbed from the gastrointestinal tract and transferred in the liver. Inside the liver, the metronidazole is metabolized and then its by-products are excreted through the feces and urine.

Metronidazole takes effect within 1 to 2 hours of administration. Its effects cannot be noticed immediately. However, gradual improvement should be expected after few days of use.

As stated, metronidazole is powerful only against anaerobic bacteria. Therefore, when used to treat mixed-bacterial infections, it is combined with other antibiotics. Metronidazole is compatible with a plethora of antibiotics including aminoglycosides, penicillin and certain members of the cephalosporins group.

Side Effects of Flagyl in Dogs

The side-effects caused by metronidazole are usually related with the drug’s bad taste or related to gastrointestinal upset. The metronidazole side-effects can be categorized as common and rare.

Commonly observed side-effects include:

  • Excessive salivation
  • Pawing at the mouth
  • Regurgitating
  • Gagging
  • Vomiting
  • Nausea
  • Decreased appetite.

The group of rare side-effects includes:

  • Diarrhea
  • Lethargy
  • Depression
  • Weakness
  • Blood in the urine
  • Dark urine
  • Low white blood cells count
  • Liver failure.

In extremely rare cases, it is possible for some dogs to develop allergies when given metronidazole. The allergic reaction clinically manifests with hives, rashes, fast heartbeat and impaired breathing. It should be noted that allergic reactions are potentially life-threatening conditions that require immediate veterinary attention.

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Also in extremely rare circumstances, metronidazole can cause a skin condition called cutaneous vasculitis. In this disorder, the blood vessels in the skin become inflamed which is followed by bruising, swelling, scaling, hair loss and development of skin bumps.


Metronidazole should not be used, or used with extra caution in the following situations:

  • In pregnant female dogs, particularly during the first trimester. This is because metronidazole has been reported to cause birth defects.
  • In nursing bitches because metronidazole is excreted through the breast milk.
  • In young puppies
  • In dogs with kidney or liver problems, metronidazole should not be used or it should be used at much lower doses.
  • In dogs with neutropenia (low white blood cells count).
  • In dogs with neurological disorders such as seizures.
  • In dogs on blood thinners (anticoagulants such as warfarin or coumarin) because it elevates the prothrombin time.

Flagyl for Dogs Drug interactions

Metronidazole is known to interact:

  • With warfarin or coumarin anticoagulants it leads to increased prothrombin time
  • With phenobarbital and phenytoin which increase the metronidazole’s metabolism
  • With certain chemotherapy drugs
  • With certain gastroprotectants such as cimetidine
  • With sedatives.

Flagyl Overdose or Toxicity

Generally speaking, metronidazole has a wide safety margin. However, long-term use (over 12 days) of moderate to high doses (over 62 milligrams per kilogram per day) may lead to acute toxicity. Acute metronidazole toxicosis in dogs can also be caused by accidental overdoses.

In such cases, the metronidazole has neurotoxic effects which manifest with:

  • Lack of appetite
  • Intermittent vomiting
  • Agitation
  • Extreme lethargy
  • Lack of muscle control
  • Lack of coordination (ataxia)
  • Abnormal gait and posture
  • Tremors
  • Seizures
  • Eye twitching
  • Head tilt
  • Proprioceptive deficits
  • Rigidity
  • Stiffness
  • Paralysis on all four legs

Signs of gastrointestinal upset such as vomiting, diarrhea and anorexia are also likely to develop. The best way to diagnose metronidazole toxicosis is by administering diazepam intravenously. Namely, the diazepam causes an immediately visible but transient improvement of the clinical signs.

The treatment of metronidazole toxicity includes:

  • Discontinuation of the drug – because of the drug’s relatively short half-life, the organism gets rid of the drug and the neurological signs start to diminish quickly. However, it may take between several days to up to 2 weeks for the neurological signs to disappear completely.
  • Diazepam administration – the diazepam is administered orally at a dose of 0.43 milligrams per kilo every 8 hours usually for about 3 days. The diazepam alleviates the neurological symptoms thus shortening the recovery period.
  • Fluid therapy – to keep the dog well hydrated and nutritionally satiated, fluid therapy also helps flush the kidneys thus promoting faster metronidazole elimination.
  • Symptomatic and supportive care – individually tailored based on the patient’s needs.

For proper observation and treatment, dogs with metronidazole toxicosis should be hospitalized for at least 24 hours. However, the exact length of the hospitalization period depends on the severity of the clinical manifestation and the patient’s response to treatment.

All in all, the recovery period is long but fortunately, long-lasting complications are extremely rare. It should be noted that the cost of treating metronidazole toxicity ranges between $200 and $1000.

About the Author

Dr. Ivana Crnec is a graduate of the University Sv. Kliment Ohridski’s Faculty of Veterinary Medicine in Bitola, Republic of Macedonia.

ivana crnec

She currently practices as a veterinarian in Bitola and is completing her postgraduate studies in the Pathology of Domestic Carnivores at the Faculty of Veterinary Medicine in Zagreb, Croatia.

Ivana’s research has been published in international journals, and she regularly attends international veterinary conferences.

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