Dr. Mark is a veterinarian. He has been working with dogs for more than 40 years.
What Is Metronidazole and How Does It Work?
Metronidazole is a nitroimidazole antibiotic effective in eliminating anaerobic bacterial infections and some protozoal parasitic infections like Giardia, Entamoeba, Balantidium, and Trichomonas. It is available as Flagyl and Metrogel and is also sold as a generic.
The medication is bactericidal. It is toxic to anaerobic bacterial infections, but if the veterinarian suspects a mixed aerobic/anaerobic infection, it is combined with amoxicillin, ciprofloxacin, clindamycin, and many other antibiotics.
This antibacterial medication is not FDA-approved for dogs and cats but is a common extra-label drug to treat diarrhea. (An extra-label drug is a medication that has not been FDA approved but is used to treat pets.)
What Is Metronidazole Used for in Dogs?
Metronidazole is a useful antibiotic and has several uses in dogs:
- Protozoal infections (Giardia, Entamoeba, rarely Balandtidium)
- Bacterial infections (especially in the oral cavity)
- Inflammatory bowel disease (because of its immune-modulating activity)
- Colitis (often used when secondary to other antibiotics)
- Diarrhea (especially when the cause cannot be determined or is caused by Clostridium perfringens) (1)
- Small-intestine bacterial overgrowth (secondary to pancreatic insufficiency)
- Liver disease (hepatic encephalopathy)
Does Metronidazole Stop Diarrhea in Dogs
Some causes of diarrhea can be treated with metronidazole. If the cause of the diarrhea cannot be determined, metronidazole is sometimes used effectively.
Does Metronidazole Stop Diarrhea in Cats?
This medication can be used to treat diarrhea caused by Giardia in cats. Trichomonas infections are more likely to respond to ronidazole but are sometimes treated with metronidazole.
Metronidazole can also be used for inflammatory bowel disease in cats. However, many cats with diarrhea also respond to dietary changes and/or benefit from probiotics, which cause an improvement in the bacterial biome of the GI tract. Those are great alternatives to try before turning to metronidazole. (2)
How Does Metronidazole Make Dogs Feel?
Metronidazole will treat some infections, but it will often make dogs feel worse before they are better. Some dogs just become listless and nauseous after treatment whereas others have vomiting, diarrhea, and excessive drooling.
The most serious side effects of metronidazole are neurological. Dogs can develop ataxia (loss of coordination and muscle control), and have tremors, seizures, head tilt, and eye twitching. These neurologic symptoms can present even after a brief duration of treatment. (3)
If you see any signs of toxicity after giving your dog or cat metronidazole, contact the prescribing veterinarian immediately.
Other Uses for Metronidazole in Veterinary Medicine
Metronidazole is sometimes used for other infections in dogs and cats:
- Meningitis: This medication crosses the blood-brain barrier, and levels in the cerebrospinal fluid are almost as high as those in the blood. (4)
- Stomatitis in Cats: Often used with corticosteroids.
- Feline Acne: Used as a gel in cases where the veterinarian does not see a response to other antibiotic creams.
In the past, this drug was also used for tear staining in white dogs.
How Is Metronidazole Given?
Metronidazole can be given as an injection or topically, but it is most commonly given orally. It is available in capsules, pills, and a liquid made up by compounding pharmacies.
It is usually given with food but tastes bitter, and some dogs and cats will refuse to take it. A liquid suspension with flavoring added (beef, chicken, tuna, salmon, peanut butter, etc.) is available. If you need to use the liquid, discuss it with your veterinarian because if your pet is on a novel food trial for diarrhea it will need to be avoided.
Some veterinarians recommend that the tablet be enclosed in a gel capsule to disguise the taste.
What to Do If Your Pet Misses a Dose
If you forget to give your pet a dose of metronidazole, skip that dose and give the next dose at the regular time. Do not give your dog or cat two doses at one time.
There are several apps available to alert you when it is time to dose your dog or cat.
How Long Does It Take for Metronidazole to Work?
Metronidazole is absorbed quickly after giving it by mouth and is present in the blood within about 20 minutes. (5) It usually takes one or two hours to start working, however, and improvements may not be noticed for several days.
Be sure to give the medication for the prescribed number of days even if your dog’s symptoms are better in only a day or two.
Potential Metronidazole Side Effects
The most common side effects of metronidazole are mild and mostly due to its bad taste:
- oral irritation (paws at the mouth)
- excessive salivation
- not eating
Some side effects are less common and more serious:
- straining to defecate (appears like constipation)
- liver failure
- dark urine
Neurological problems happen even more rarely, usually when dogs are on a high dose or are being treated for a long time: (3)
- head tilt
- eye twitching
If you see any of the signs of toxicity after giving your dog or cat metronidazole, contact the prescribing veterinarian immediately. Dogs can be treated with diazepam and usually recover in less than 2 days. (6)
Can Metronidazole Cause My Dog to Be Constipated?
Metronidazole is more likely to cause diarrhea than constipation. When some dogs have diarrhea, though, they will strain to defecate even when they no longer have to go. This is often reported as constipation even though it is not.
Risk Factors and Special Precautions
One study found brain damage in several dogs treated with metronidazole at higher doses and recommended that this medication not be used at over 40 mg/kg in a 24-hour period. (3) Another reported toxicity even at normally prescribed levels when given long-term.
Metronidazole has also caused birth defects so should not be used in pregnant pets. (7) It is also excreted in breast milk so should not be used in lactating dogs and cats.
Since metronidazole is metabolized in the liver and excreted in the urine, it is not normally used in pets suffering from liver failure or kidney disease.
Known Drug Interactions
Metronidazole is known to interact with several drugs:
- Cimetidine: It may be metabolized more slowly, so an overdose is more likely.
- Coumarin (Warfarin): Dogs taking this medication with metronidazole have an increased chance of bleeding.
- Cyclosporine: This potent immunosuppressant has higher levels when given with metronidazole.
- Phenobarbital and Phenytoin: These medications used for dogs with seizures also decrease metronidazole levels.
There are many other medications that can affect metronidazole. If your dog is on any herbal supplements, medicated foods, or traditional Chinese medicine, inform your veterinarian when metronidazole is prescribed.
Can Dogs Overdose on Metronidazole?
Dogs can overdose on metronidazole in two ways. The first is by giving too high a dose at one time (for example if you forget to give a dose and give two doses at the next scheduled time). The second type of overdose happens when a dog gets the normal dose but for a longer period of time than normal.
Symptoms of Overdose from Metronidazole Are:
- not eating
- head tilt
If you accidentally give your dog an overdose or see signs that your dog has an overdose from extended use, you should contact your veterinarian immediately. If they are not available, take your dog to an emergency veterinarian. Metronidazole toxicity will usually clear up in less than two days if it is treated. (6)
Since a common dose rate is 25 mg/pound for about 5 days, the cost of the medication for a short treatment period is about $20 to $40 or less. If your dog is going to be treated for a longer period of time, it is more cost-effective to buy a larger supply. An entire month’s supply for a small dog will usually only cost about $50.
Where Can I Buy Metronidazole for Dogs
Many veterinarians and most pharmacies carry generic metronidazole and Flagyl; it is only available with a prescription. You can also order metronidazole online.
How to Store Metronidazole
Metronidazole should be stored in the original labeled container that you purchased it in. It can be kept at room temperature, but keep out of the direct sun, away from heat sources, and out of reach of children.
Liquid metronidazole from a compounding pharmacy should also be stored in the original container at room temperature.
There are many alternatives to metronidazole, and some veterinarians argue that the medication is no longer effective because of antibiotic resistance. The alternatives are:
- Antibiotics: Dogs with diarrhea secondary to giardia may no longer have the infection but still have diarrhea. Recommendations are to stop the medication for Giardia, let the dog rest, and then treat with antibiotics for secondary bacterial infections. Antibiotics used for diarrhea include ronidazole (8), azithromycin (9), and tylosin. (10)
- Antiparasitics: Fenbendazole is a drug used to treat internal parasites. Research shows that it is just as effective as metronidazole in treating Giardia infections, (11) but neither drug cleared up all of the infections the first time the dogs were treated. Albendazole has also been used effectively. (12)
- Probiotics: By giving dogs probiotics, the bacteria that are needed for the GI to function effectively are restored. (13) Some dogs have diarrhea secondary to antibiotics and others have a proliferation of a type of bacteria that are causing the diarrhea. Probiotics like Saccharomyces boulardii can restore that balance.
- Fecal Transfer: Dogs that have been treated with metronidazole and other antibiotics have an altered microbiome, (14) and this newer area of research aims to re-establish the normal bacteria in the gut. Researchers now state that improvement of the microbiome is the only effective way to treat problems like inflammatory bowel disease. (15)
- Supplements and Food: Prebiotic fibers may help change the microbiome and make diarrhea less likely. Anecdotal evidence of the effectiveness of carvacrol (a phytonutrient made from oregano) shows that it is another alternative for bacteria (16) and viruses. (17) Herbal therapies are sometimes added to food.
(1) Langlois DK, Koenigshof AM, Mani R. Metronidazole treatment of acute diarrhea in dogs: A randomized double blinded placebo-controlled clinical trial. J Vet Intern Med. 2020 Jan;34(1):98-104. https://pubmed.ncbi.nlm.nih.gov/31742807/
(2) Simpson KW. The Role of the Microbiota in Feline Inflammatory Bowel Disease. August's Consultations in Feline Internal Medicine, Volume 7. 2016:109–16. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7152113/
(3) Tauro A, Beltran E, Cherubini GB, Coelho AT, Wessmann A, Driver CJ, Rusbridge CJ. Metronidazole-induced neurotoxicity in 26 dogs. Aust Vet J. 2018 Dec;96(12):495-501. https://pubmed.ncbi.nlm.nih.gov/30478843/
(4) Hoffmann HG, Förster D, Muirhead B. Metronidazol-Konzentration im Liquor bei gering entzündeten Hirnhäuten [Metronidazole concentration of the cerebrospinal fluid from slightly inflamed meninges]. Arzneimittelforschung. 1984;34(7):830-1. https://pubmed.ncbi.nlm.nih.gov/6541920/
(5) Datapharm metronidazole tablets insert, https://www.medicines.org.uk/emc/product/5882/smpc#gref
(6) Evans J, Levesque D, Knowles K, Longshore R, Plummer S. Diazepam as a treatment for metronidazole toxicosis in dogs: a retrospective study of 21 cases. J Vet Intern Med. 2003 May-Jun;17(3):304-10. https://pubmed.ncbi.nlm.nih.gov/12774970/
(7)Rebuelto M, Loza ME. Antibiotic Treatment of Dogs and Cats during Pregnancy. Vet Med Int. 2010 Dec 14;2010:385640. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3021871/
(8) Fiechter R, Deplazes P, Schnyder M. Control of Giardia infections with ronidazole and intensive hygiene management in a dog kennel. Vet Parasitol. 2012 Jun 8;187(1-2):93-8. https://pubmed.ncbi.nlm.nih.gov/22240238/
(9) Zygner W, Jaros D, Gójska-Zygner O, Wedrychowicz H. Azithromycin in the treatment of a dog infected with Giardia intestinalis. Pol J Vet Sci. 2008;11(3):231-4. https://pubmed.ncbi.nlm.nih.gov/18942546/
(10) Kilpinen S, Spillmann T, Syrjä P, Skrzypczak T, Louhelainen M, Westermarck E. Effect of tylosin on dogs with suspected tylosin-responsive diarrhea: a placebo-controlled, randomized, double-blinded, prospective clinical trial. Acta Vet Scand. 2011 Apr 14;53(1):26. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3084160/
(11) Ciuca L, Pepe P, Bosco A, Caccio SM, Maurelli MP, Sannella AR, Vismarra A, Cringoli G, Kramer L, Rinaldi L, Genchi M. Effectiveness of Fenbendazole and Metronidazole Against Giardia Infection in Dogs Monitored for 50-Days in Home-Conditions. Front Vet Sci. 2021 Mar 26;8:626424. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8032893/
(12) Barr SC, Bowman DD, Heller RL, Erb HN. Efficacy of albendazole against giardiasis in dogs. Am J Vet Res. 1993 Jun;54(6):926-8. https://pubmed.ncbi.nlm.nih.gov/8323064/
(13) Jensen AP, Bjørnvad CR. Clinical effect of probiotics in prevention or treatment of gastrointestinal disease in dogs: A systematic review. J Vet Intern Med. 2019 Sep;33(5):1849-1864. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6766488/
(14) Igarashi H, Maeda S, Ohno K, Horigome A, Odamaki T, Tsujimoto H. Effect of oral administration of metronidazole or prednisolone on fecal microbiota in dogs. PLoS One. 2014 Sep 17;9(9):e107909. https://pubmed.ncbi.nlm.nih.gov/25229475/
(15) Niina A, Kibe R, Suzuki R, Yuchi Y, Teshima T, Matsumoto H, Kataoka Y, Koyama H. Improvement in Clinical Symptoms and Fecal Microbiome After Fecal Microbiota Transplantation in a Dog with Inflammatory Bowel Disease. Vet Med (Auckl). 2019 Dec 2;10:197-201. https://pubmed.ncbi.nlm.nih.gov/31819862/
(16) Katsoulos PD, Karatzia MA, Dovas CI, Filioussis G, Papadopoulos E, Kiossis E, Arsenopoulos K, Papadopoulos T, Boscos C, Karatzias H. Evaluation of the in-field efficacy of oregano essential oil administration on the control of neonatal diarrhea syndrome in calves. Res Vet Sci. 2017 Dec;115:478-483. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7185441/
(17) Sánchez C, Aznar R, Sánchez G. The effect of carvacrol on enteric viruses. Int J Food Microbiol. 2015 Jan 2;192:72-6. https://pubmed.ncbi.nlm.nih.gov/25310265/
This article is not meant to substitute for diagnosis, prognosis, treatment, prescription, or formal and individualized advice from your veterinarian. Animals exhibiting signs and symptoms of distress should be seen by a veterinarian immediately.
© 2022 Mark dos Anjos DVM