Metronidazole Side Effects in Dogs

Your veterinarian might prescribe metronidazole, an antibiotic marketed under the brand name Flagyl, if your dog suffers from diarrhea due to certain bacterial infections or parasite infestation. Because it can penetrate your dog's blood-brain barrier, it's also prescribed for infections of the central nervous system. While the drug is generally well-tolerated, some dogs will experience side effects, with the odds increasing with the dosage amount. Call your vet if your dog exhibits any side effects while taking this medication.

Veterinarian with dog in examination room
Your vet likely will recommend that you administer metronidazole along with food.
credit: XiXinXing/XiXinXing/Getty Images

How it Works

Your vet might prescribe metronidazole in conjunction with other antibiotics, since this medication works against anaerobic bacteria -- those not requiring oxygen -- and other antibiotics are necessary to combat bacterial mixes. Internal abscesses are among the conditions that metronidazole can treat, working by distorting DNA synthesis and killing the bacteria. Metronidazole also has the ability to penetrate bone, so your vet might prescribe it if your dog is diagnosed with a dental infection or osteomyelitis.

Common Side Effects

Metronidazole's most common side effects are related primarily to the gastrointestinal system. Some dogs might experience appetite loss, nausea or vomiting after metronidazole administration. Hypersalivation -- or excessive drooling -- is also common. Your dog might paw at his mouth as well.

Uncommon Side Effects

Although gastrointestinal in nature, diarrhea is a relatively uncommon metronidazole side effect. Some dogs become lethargic or depressed while on the drug. Low white blood cell counts can occur. A dog's urine might change color, becoming significantly darker, or contain blood. One relatively rare, but serious, side effect is liver failure.

Metronidazole Neurotoxicity

Dogs on high or long-term doses of metronidazole might develop neurological symptoms relating to drug toxicity. Symptoms include staggering, seizures, stiffness, head tilt, rapid back and forth eye movement and strange postures. If your dog show signs of neurotoxicity, take him to the vet immediately. Fortunately, most dogs recover from their bout with metronidazole neurotoxicity within two weeks of stopping the drug. Dogs treated with diazepam -- marketed under the trade name Valium -- recovered much faster, usually within a few days.

Precautions and Contraindications

Pregnant or nursing dogs or young puppies should not receive metronidazole. If your dog is diagnosed with liver or kidney disease, he shouldn't receive this medication. If your dog is epileptic and takes phenobarbital, the effects of metronidazole will be reduced. Metronidazole reacts with a variety of drugs. Let your vet know of any medications or over-the-counter supplements your dog receives.