Metronidazole, also known as Flagyl, is a multi-purpose antibiotic often used in the treatment of dogs. While it’s not yet approved for use in animals by the FDA, it is commonly prescribed by vets for the treatment of Giardia and bacterial infections. It works by destroying and preventing the creation of DNA in infecting organisms.

When treating Giardia a higher dose will be used which can increase the risk of side effects. Because bad reactions to metronidazole can be serious it’s usually better to use Panacur in these cases.

Recommended Dosage

Note: Metronidazole is a prescription medicine and should never be used unless it has been prescribed by a licensed vet. Stay safe by following the vet’s recommendations at all times.

The correct dosage will depend on the condition being treated. When treating Giardia, metronidazole is often given at 7 to 11 mg/lb every 24 hours, but up to 29.5 mg/lb is possible in some cases when the vet deems it necessary. For anaerobic infections 11 – 22 mg/lb given twice daily is common. Always follow your vet’s advice.

The chart below shows an 11 mg/lb dose which can treat both bacterial infections and Giardia.

Flagyl dose diagram
Dog’s Weight In Pounds: Dosage In Mg:

Most tablets contain 250 or 500 mg of active ingredient per pill. There are also extended-release capsules made for humans, but because dogs have a shorter digestive tract than us these do not work well and usually pass through the system only partially digested.

It may be injected by your vet in cases of sepsis or when oral administration is impossible due to blocked or constricted airways.

Example: A 25 lb dog could be given 275 mg of Flagyl per dose.

Is It Safe?

This medicine is safe in most cases but is not yet FDA-approved for veterinary use and can cause serious adverse effects in rare cases. It is thought to affect fetuses so it’s best not to give it to a pregnant or nursing dog, and is also unsuitable for very young puppies.

For Giardia infections we recommend using Panacur which tends to be safer. Unlike metronidazole, Panacur has been approved by the FDA for veterinary use.

Treatment may not be suitable for those suffering from the following conditions:

  • Liver disease
  • Neutropenia
  • Seizure disorders

If you can’t avoid giving it to a dog with liver disease the dose can be cut in half. In some cases animals treated with the drug could experience neurological effects.

If treatment is not suitable for your dog you can ask the vet about other antibiotics such as Baytril (enrofloxacin), though Baytril is not effective for treating anaerobic infections.

Guidelines For Safe Use

To safely use metronidazole you should follow these guidelines:

  • Always speak to a vet before use
  • Tell your vet about any other medicine your pet is taking
  • Tell the vet about any medical conditions your dog is suffering from
  • Avoid use in nursing or pregnant animals

Be cautious when using the drug for extended periods of time as adverse effects are more common with long-term use. Contact a vet right away if your pet suffers any neurotoxic effects (e.g. paralysis in all four legs caused by peripheral neuropathy).


This antibiotic is effective against:

  • Anaerobic infections
  • Trichomonas (parasite)
  • Giardia (not recommended)
  • Inflammatory Bowel Disease (IBD)
  • Sepsis

For treating trichomonas lower amounts of medicine are often used.

Can Flagyl cure all infections caused by bacteria?

While effective against most anaerobes, certain forms of actinobacteria are resistant to the drug. In these cases you will need to seek out a different solution.

Side Effects In Dogs

During treatment your dog may experience the following:

More common

  • Nausea
  • Vomiting
  • Diarrhea
  • Fatigue
  • Loss of appetite
  • Blood in urine
  • Fever


  • Thrombocytopenia

Incidence Unknown

  • Liver damage
  • Neurotoxic effects
  • Neutropenia
  • Insomnia

Call a vet for immediate advice if severe side effects or an allergic reaction occurs. Look out for rashes, hives, swelling, difficulty breathing and a rapid heartbeat which all point to an allergic reaction. If left untreated allergies can quickly become dangerous and even life-threatening.


Dogs who receive too much of this medicine, or those treated for a longer period of time, could experience these effects:

  • Weight loss
  • Vomiting
  • Irregular heartbeat
  • Tremors
  • Seizures
  • Confusion
  • Stiffness
  • Abnormally slow heartbeat
  • Dilated pupils

In the event that a dog accidentally receives too much medicine you should seek emergency veterinary attention. You can also call the ASPCA Pet Poison hotline on (888) 426-4435.

Further Information

For more information, you may find this video helpful:



Dr. K. Schunk
Plumb’s Veterinary Drug Handbook (sixth edition)