Furazolidone
Furazolidone is a synthetic **nitrofuran-derivative** antibacterial and antiprotozoal agent. Historically, it was utilized in small animal medicine as a second-line treatment for enteric infections, particularly those caused by *Giardia*, *Coccidia*, and various bacterial enteropathogens (e.g., *Campylobacter*, *Salmonella*, *E. coli*). > **Clinical Pearl:** Due to the withdrawal of human commercial products in many countries and the strict FDA prohibition of its extralabel use in food-producing animals (due to carcinogenic concerns associated with nitrofurans), its use in modern veterinary practice is exceedingly rare. Safer and more readily available alternatives like fenbendazole and metronidazole are now the standard of care for giardiasis and similar enteric infections.
Mechanism: Furazolidone acts as a prodrug. Inside susceptible organisms, it is reduced by **bacterial flavoproteins** (nitroreductases) โ highly reactive intermediate compounds are formed โ these intermediates cause severe oxidative damage to **ribosomal proteins, DNA, and RNA**, disrupting cellular metabolism and inhibiting protein synthesis. * **Antiprotozoal mechanism:** While not fully elucidated, it likely involves similar oxidative damage to essential protozoal enzymes. * **MAO Inhibition:** Furazolidone also acts as a **monoamine oxidase inhibitor (MAOI)** in the mammalian host, which can lead to significant drug and dietary interactions (e.g., with tyramine-rich foods or sympathomimetic drugs).
Dosing by species
- Giardia ยท 4 mg/kg PO twice daily (q12h) ยท PO ยท q12h ยท 7-10 days ยท If re-treatment is required, elevated dosages or lengthened treatment regimens may provide better results.
- Giardia ยท 4 mg/kg, PO twice daily (q12h) ยท PO ยท q12h ยท 7 days
- Cystoisospora spp. ยท 8-20 mg/kg PO q12-24h ยท PO ยท q12-24h ยท 5 days
- Coccidiosis ยท 8-20 mg/kg PO once daily ยท PO ยท q24h ยท 7 days
- Giardiasis ยท 4 mg/kg PO q12h ยท PO ยท q12h ยท 5-10 days
- Amebiasis ยท 2.2 mg/kg PO q8h ยท PO ยท q8h ยท 7 days
- Amebic colitis ยท 2.2 mg/kg PO q8h ยท PO ยท q8h ยท 7 days
- Coccidiosis ยท 8-20 mg/kg PO for one week ยท PO ยท Unknown ยท 1 week
- Giardia ยท 4 mg/kg PO q12h ยท PO ยท q12h ยท 5 days
- Susceptible infections ยท 4 mg/kg PO three times daily ยท PO ยท q8h
- Amebic colitis ยท 2.2 mg/kg PO q8h ยท PO ยท q8h ยท 7 days
Routes of administration
Contraindications
- Known hypersensitivity to furazolidone or other nitrofurans
- Food-producing animals (FDA strictly prohibits extralabel use)
- Pregnant queens (per some references)
Adverse effects
- Anorexia
- Vomiting
- Abdominal cramping
- Diarrhea
- Urine discoloration (dark yellow to brown)
Drug interactions
- Alcohol ยท May cause a disulfiram-like reaction.
- Amitraz ยท Concurrent use not recommended due to MAO inhibition; dangerous hypertension could occur.
- Buspirone ยท Concurrent use not recommended due to MAO inhibition; dangerous hypertension could occur.
- Selegiline ยท Concurrent use not recommended due to MAO inhibition; dangerous hypertension could occur.
- Sympathomimetic amines (phenylpropanolamine, ephedrine, etc.) ยท Concurrent use not recommended due to MAO inhibition; dangerous hypertension could occur.
- Tricyclic antidepressants ยท Concurrent use not recommended due to MAO inhibition; dangerous hypertension could occur.
- High tyramine foods (fish, poultry) ยท Concurrent use not recommended due to MAO inhibition; dangerous hypertension could occur.
Monitoring
- Efficacy (stool exams for parasitic infections)
Overdose
Moderate overdoses are unlikely to cause significant toxicity. Gut emptying may be considered for large overdoses.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.