Furazolidone

Overview

Furazolidone is a nitrofuran antimicrobial agent with both antibacterial and antiprotozoal activity. It has been used in dogs and cats primarily for the treatment of enteric infections, although it is generally considered a second-line option and is not widely available in some regions.

The drug is mainly utilized for gastrointestinal infections, and its clinical use is often limited by availability and potential drug interactions. It is administered orally and is typically reserved for cases where first-line therapies are ineffective or not suitable.

Mechanism of Action (MOA): Furazolidone exerts bactericidal effects by interfering with bacterial enzyme systems, leading to disruption of essential cellular processes. Its mechanism against protozoa is less clearly defined. In addition, its metabolites inhibit monoamine oxidase (MAO), which contributes to its interaction profile with several drugs and substances.

 US unavailable / Furoxone withdrawn: Furazolidone is NO LONGER commercially available in the United States. Outside the US, international trade names include Enterolidon, Exofur, Furasian, Furion, Furoxona, Fuxol, Giarcid, Giardil, Giarlam, Neo Furasil, Nifuran, Novafur, Salmocide, and Seforman.

Spectrum: Furazolidone has activity against Giardia spp, Vibrio cholerae, Trichomonas spp, Coccidia spp, and many strains of Escherichia coli, Enterobacter spp, Campylobacter spp, Salmonella spp, and Shigella spp. Not all strains are sensitive, but resistance is usually LIMITED and develops SLOWLY — useful for refractory cases when culture supports.

 PK + DI consequence: Published information on absorption is CONFLICTING. Pigmented metabolites in urine indicate the drug IS absorbed to some extent. Reportedly, furazolidone distributes into the CSF. Absorbed furazolidone is rapidly metabolized in the LIVER, and the majority is eliminated in the URINE. Practical implication: because absorption is variable and poor, the clinical significance of the many MAO-related drug interactions remains unclear — but vets must still avoid concurrent MAOIs and tyramine-rich foods given the high consequence of an interaction.

  Food-animal use PROHIBITED: EXTRA-LABEL USE of furazolidone is PROHIBITED in food-producing animals. extra-label withdrawal-interval estimates in non-food-animal companion species.

Local enteric action only: furazolidone is used ONLY for LOCAL treatment of intestinal parasites; it is NOT used for systemic therapy. Combined with the conflicting / generally poor absorption profile, this means furazolidone has no role in systemic bacterial or protozoal infections — reserve strictly for enteric / luminal indications.

Indications

Furazolidone is used in dogs and cats primarily for the treatment of gastrointestinal infections caused by susceptible bacteria and protozoa. It is generally considered a second-line option and is reserved for cases where more commonly used therapies are ineffective or not suitable.

• Enteric bacterial infections: May be used for gastrointestinal infections caused by susceptible organisms such as Escherichia coli, Salmonella, Campylobacter, and related enteric pathogens.
• Protozoal infections (e.g., giardiasis): Has activity against Giardia species and may be used in selected cases when standard treatments are not effective or tolerated.
• Mixed enteric infections: Useful in cases where both bacterial and protozoal pathogens are suspected or confirmed, particularly in refractory or recurrent gastrointestinal disease.
•  Fenbendazole first-line: For canine + feline GIARDIASIS, FENBENDAZOLE is preferred. Furazolidone is reserved for cases refractory or intolerant to fenbendazole and/or metronidazole.
• Less-known activity — Trichomonas + Coccidia + V. cholerae: furazolidone also has activity against Trichomonas spp, Coccidia spp, and Vibrio cholerae — relevant for less-common refractory protozoal cases (e.g. trichomoniasis in cats, coccidiosis in puppies / kittens) where first-line agents have failed.

Dosage (Reference)

Dog

In dogs, furazolidone is used extra-label as a second-line agent for enteric protozoal infection — principally giardiasis that has failed or is not tolerated on fenbendazole or metronidazole. Dedicated canine studies are lacking, so response should be confirmed on a follow-up faecal examination.

Cat

In cats, furazolidone is used extra-label for the treatment of protozoal infections such as giardiasis. Treatment duration and response may vary, and adjustment may be required in refractory cases.

Warnings & Precautions

Furazolidone should be used with caution in dogs and cats due to its potential for drug interactions and limited safety data. Careful patient selection and awareness of its pharmacologic effects are important when considering its use.

• Hypersensitivity: Contraindicated in animals with known hypersensitivity to furazolidone or other nitrofuran compounds.
• Monoamine oxidase (MAO) inhibition: Furazolidone inhibits MAO, which may predispose patients to significant drug and dietary interactions; concurrent use with interacting agents should be avoided or carefully monitored.
• Drug interaction potential: Use cautiously when administered with drugs that affect serotonin, catecholamines, or blood pressure due to the risk of adverse reactions.
• Pregnancy: Safety has not been established; avoid use in pregnant animals (particularly queens) unless benefits clearly outweigh potential risks.
• Lactation: It is unknown whether the drug is excreted in milk; use cautiously in nursing animals.
• Limited clinical data: Due to incomplete pharmacokinetic and safety information, use should generally be reserved for cases where standard therapies are not appropriate.
• Lab caveat — false-positive Clinitest: Furazolidone may cause a FALSE-POSITIVE urine glucose determination when using the CUPRIC SULFATE solution test (e.g. Clinitest®). Use a urine dipstick (glucose oxidase) for accurate urine glucose during therapy, especially when the patient is a diabetic-suspect cat or polyuric.

Drug Interactions

Furazolidone has a high potential for drug interactions in dogs and cats, primarily due to its inhibition of monoamine oxidase (MAO). This can lead to serious effects such as hypertension or altered drug responses when combined with certain medications or substances.

• Ethanol: May cause a disulfiram-like reaction (e.g., vomiting, flushing); concurrent use should be avoided.
• MAO-interacting drugs: Concurrent use is not recommended due to the risk of dangerous hypertension.
• Serotonergic agents (e.g., fluoxetine, sertraline, trazodone, mirtazapine, cyproheptadine): Increased risk of serotonin-related effects and hypertension.
• Tricyclic antidepressants (e.g., amitriptyline, clomipramine): May potentiate hypertensive reactions when combined.
• Sympathomimetic amines (e.g., ephedrine, phenylpropanolamine): Increased risk of excessive sympathetic stimulation and hypertension.
• Selegiline: Additive MAO inhibition may result in severe adverse effects; avoid concurrent use.
• Linezolid and methylene blue: Additional MAO inhibition increases the risk of serious interactions.
• Buspirone and amitraz: Potential for exaggerated pharmacologic effects due to MAO inhibition.
• Tyramine-containing foods: May precipitate hypertensive reactions; avoid concurrent exposure.
• Specific tyramine foods to avoid: FISH, POULTRY, SMOKED MEATS, and AGED CHEESES. Counsel owners to withhold these throughout the treatment course. For commercial diets containing fish or poultry, consider a temporary switch to a lamb / beef-only formulation while on therapy.

Side Effects & Overdose

Side Effects

Adverse effects of furazolidone in dogs and cats are generally mild and primarily involve the gastrointestinal tract. Most effects are transient and resolve after discontinuation of therapy.

• Gastrointestinal signs: Anorexia, vomiting, abdominal cramping, and diarrhea may occur.
• Urine discoloration: May cause harmless dark yellow to brown discoloration of urine.
• Hypersensitivity reactions: Rare but possible; discontinue use if suspected.

Overdose

Limited information is available regarding overdose in dogs and cats, but moderate overdoses are unlikely to result in severe toxicity. Clinical effects are expected to be primarily gastrointestinal.

• Gastrointestinal signs: Increased severity of vomiting, diarrhea, and anorexia.
• Management: Consider gastrointestinal decontamination in large overdoses and provide supportive care as needed.
• Consultation: Veterinary poison consultation is recommended in suspected overdose cases for appropriate guidance and monitoring.

Key Notes

Practical clinical points that help guide the appropriate use of furazolidone in dogs and cats in selected cases:

• Second-line therapy: Typically reserved for cases where first-line antimicrobials or antiprotozoals are ineffective or not tolerated.
• Limited availability: The drug is not widely commercially available in some regions and may require compounding for clinical use.
• Primarily enteric action: Clinical use is mainly directed toward gastrointestinal infections, with limited relevance for systemic infections.
• Variable absorption: Systemic absorption is inconsistent, so clinical effects are largely localized within the gastrointestinal tract.
• Monitoring efficacy: Response to therapy should be evaluated with follow-up fecal examinations in parasitic infections when possible.
• Use with clinical judgment: Due to limited modern use and data, treatment decisions should be based on individual case assessment and response.
• Modern alternatives (fenbendazole / metronidazole / ronidazole / nitazoxanide): Several agents are preferred over furazolidone for canine + feline GIARDIASIS: FENBENDAZOLE, METRONIDAZOLE , RONIDAZOLE, and other emerging options. Furazolidone is now generally reserved for cases where these standard agents have failed.