Nitrofurantoin — Veterinary Vademecum Drug Reference

Dosage & Administration Guidelines

Species	Route	Dose	Frequency	Duration & Clinical Notes
Dog	PO	4-5 mg/kg	q8h	Duration: 7-14 days Notes: Administer with food to enhance absorption and reduce GI upset. Use with caution in patients with renal impairment.
Cat	PO	4-5 mg/kg	q8h	Duration: 7-14 days Notes: Administer with food. Cats may be more sensitive to GI effects; monitor for vomiting.
Horse	PO	10 mg/kg	q6-8h	Duration: 5-7 days Notes: Not commonly used; limited data. Use only if other antibiotics are not suitable.
Cattle	PO	Not established	Not established	Duration: Not established Notes: Not recommended for food animals due to lack of withdrawal data and approved formulations.
Small Ruminants	PO	Not established	Not established	Duration: Not established Notes: Not recommended for food animals due to lack of withdrawal data and approved formulations.
Rabbit	PO	5-10 mg/kg	q8-12h	Duration: 7-14 days Notes: Limited data; use with caution. Monitor for GI disturbances.
Bird/Poultry	PO	Not established	Not established	Duration: Not established Notes: Not recommended for food animals due to lack of withdrawal data and approved formulations.
Exotic/Other	PO	Variable	Variable	Duration: Variable Notes: Use with caution; consult a specialist for dosing.

Clinical Indications & Species Uses

General Indications

Treatment of uncomplicated lower urinary tract infections (cystitis) caused by susceptible bacteria.

Dog (Canine)

Treatment of uncomplicated urinary tract infections (UTIs) caused by susceptible organisms such as E. coli, Enterococcus, Staphylococcus, and Klebsiella.

Cat (Feline)

Treatment of uncomplicated urinary tract infections (UTIs) caused by susceptible organisms.

Pharmacology & Mechanism of Action

Drug Class: Antibiotic | Pharmacological Group: Nitrofuran

Mechanism of Action: Nitrofurantoin is a synthetic nitrofuran antibiotic that is bactericidal against many gram-positive and gram-negative organisms. Its mechanism involves reduction of the nitro group by bacterial flavoproteins to form reactive intermediates that damage bacterial DNA, ribosomal proteins, and other cellular components. This leads to inhibition of protein synthesis, DNA replication, and cell wall synthesis. The drug is effective primarily in the urinary tract because it is rapidly excreted in the urine, achieving high concentrations there.

Pharmacodynamics: Nitrofurantoin is active against many common urinary pathogens including Escherichia coli, Enterococcus spp., Staphylococcus spp., and some Klebsiella and Enterobacter species. It is less active against Proteus, Pseudomonas, and Serratia. The drug is bacteriostatic at low concentrations and bactericidal at higher concentrations. Its activity is enhanced in acidic urine (pH < 6). Resistance can develop, but it is less common compared to other antibiotics.

⚡ Pharmacokinetics Summary

Absorption: Nitrofurantoin is well absorbed from the gastrointestinal tract after oral administration. The presence of food enhances absorption and reduces gastrointestinal upset. In dogs, oral bioavailability is approximately 90%.

Distribution: Nitrofurantoin is widely distributed in body tissues, but it is rapidly cleared from the plasma. It does not penetrate well into tissues other than the urinary tract. It crosses the placenta and is distributed into milk.

Metabolism: Nitrofurantoin is metabolized in the liver and tissues to inactive metabolites. The metabolism is not extensive; the majority of the drug is excreted unchanged in the urine.

Excretion: Nitrofurantoin is rapidly excreted by the kidneys via glomerular filtration and tubular secretion. Approximately 30-40% of the dose is excreted unchanged in the urine, achieving high urinary concentrations. In renal impairment, excretion is reduced, leading to increased plasma levels and potential toxicity.

Half-Life: Dog: 0.5-1 hour; Cat: 0.5-1 hour; Horse: 0.5-1 hour (estimated); Cattle: 0.5-1 hour (estimated)

Bioavailability: Oral bioavailability is approximately 90% in dogs when given with food; it may be lower in other species.

Protein Binding: Nitrofurantoin is approximately 60% bound to plasma proteins.

Available Formulations & Strengths

Oral Capsule 25 mg, 50 mg, 100 mg (PO)

Oral Suspension 25 mg/5 mL (PO)

Contraindications & Clinical Warnings

Contraindications:

Hypersensitivity to nitrofurantoin or any component of the formulation.
Renal impairment (anuria, oliguria, or significant renal insufficiency) because of risk of accumulation and toxicity.
Pregnancy (especially near term) in animals, as it may cause hemolytic anemia in neonates.
Use in animals with glucose-6-phosphate dehydrogenase deficiency (rare in animals).
Do not use in animals with known peripheral neuropathy.

Warnings & Clinical Precautions:

Use with caution in animals with renal disease; adjust dose or avoid use.
May cause gastrointestinal upset; administer with food.
Prolonged use may result in superinfection with resistant organisms.
Use with caution in pregnant animals; weigh risks vs. benefits.
In horses, oral administration may cause colitis; monitor for diarrhea.
In cats, may cause methemoglobinemia at high doses; monitor for cyanosis.
Avoid use in animals with a history of liver disease.
Do not use in animals with known hypersensitivity to nitrofurans.

Adverse Effects & Reactions

Common:

Nausea
Vomiting
Diarrhea
Loss of appetite

Serious / Severe:

Pulmonary toxicity (acute or chronic) - rare but can be fatal
Peripheral neuropathy
Hepatotoxicity
Hemolytic anemia (especially in animals with G6PD deficiency)
Methemoglobinemia (in cats)

Rare:

Allergic reactions (urticaria, angioedema)
Stevens-Johnson syndrome
Aplastic anemia
Cholestatic jaundice

Key Drug Interactions

Interacting Drug / Class	Clinical Effect & Mechanism	Severity
Antacids containing magnesium trisilicate	May decrease absorption of nitrofurantoin.	Moderate
Probenecid	Decreases renal tubular secretion of nitrofurantoin, increasing plasma levels and risk of toxicity.	High
Sulfinpyrazone	Decreases renal excretion of nitrofurantoin, increasing risk of toxicity.	High
Other nephrotoxic drugs (e.g., aminoglycosides)	Additive nephrotoxicity.	Moderate

Overdose & Toxicity Management

Signs of Toxicity:

Vomiting
Diarrhea
Ataxia
Tremors
Seizures
Methemoglobinemia (especially in cats)
Respiratory distress
Hepatotoxicity

Emergency Treatment Protocol: Treatment is symptomatic and supportive. Induce vomiting if recent ingestion and animal is conscious. Administer activated charcoal to reduce absorption. Provide intravenous fluids to maintain hydration and promote diuresis. Monitor for methemoglobinemia and treat with methylene blue if severe (use with caution in cats). Provide respiratory support if needed. Monitor liver and kidney function.

Food Animal Withdrawal Times

Not approved for use in food animals in the US. No withdrawal times established. Do not use in food-producing animals.

Storage, Handling & Regulatory Information

Storage Temperature: Store at room temperature (20-25°C / 68-77°F).

Light Sensitivity: Light-sensitive — protect from direct exposure.

Handling & Special Conditions: Protect from light and moisture. Keep container tightly closed.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Approved for use in dogs and cats for urinary tract infections. Not approved for other species.

Extra-Label (Off-Label) Use: In the US, extra-label use in food animals is prohibited due to lack of withdrawal data and potential for residues. In non-food animals, extra-label use is permitted under AMDUCA with appropriate veterinary oversight.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Nitrofurantoin is a valuable antibiotic for treating uncomplicated lower urinary tract infections in dogs and cats. It is not effective for systemic infections due to poor tissue penetration. It should be used only when culture and sensitivity confirm susceptibility. Administer with food to reduce GI upset. Use with caution in patients with renal impairment, as it may accumulate and cause toxicity. In cats, monitor for methemoglobinemia, especially at higher doses. Not recommended for food animals due to lack of withdrawal data. Always follow label directions and consult a veterinarian for appropriate dosing and duration.

References & Literature

📚 Plumb's Veterinary Drug Handbook
📚 Papich Veterinary Pharmacology and Therapeutics
📚 Compendium of Veterinary Products (CVP)