Nystatin — Veterinary Vademecum Drug Reference

Dosage & Administration Guidelines

Species	Route	Dose	Frequency	Duration & Clinical Notes
Dog	PO	100,000-200,000 units per dog	q8h	Duration: 7-14 days Notes: For oral candidiasis, may also be used as a suspension for swabbing the oral cavity.
Cat	PO	100,000-200,000 units per cat	q8h	Duration: 7-14 days Notes: For oral candidiasis, may also be used as a suspension for swabbing the oral cavity.
Horse	PO	500,000-1,000,000 units per horse	q12h	Duration: 7-14 days Notes: For gastric candidiasis in foals, use 100,000-250,000 units per foal q6-8h.
Cattle	PO	500,000-1,000,000 units per calf	q12h	Duration: 5-7 days Notes: For mycotic rumenitis, administer directly into the rumen via stomach tube.
Small Ruminants	PO	100,000-200,000 units per animal	q12h	Duration: 7-14 days Notes: For oral candidiasis, may be used as a suspension.
Rabbit	PO	100,000 units per kg	q12h	Duration: 7-14 days Notes: For gastrointestinal candidiasis, may be administered via oral syringe.
Bird/Poultry	PO	100,000-200,000 units per kg	q12h	Duration: 5-7 days Notes: For crop candidiasis, may be administered directly into the crop.
Exotic/Other	PO or Topical	100,000 units per kg	q12h	Duration: 7-14 days Notes: For reptiles, use topical application on lesions; for small mammals, oral administration.

Clinical Indications & Species Uses

General Indications

Treatment of superficial candidal infections of the skin, mucous membranes, and gastrointestinal tract

Dog (Canine)

Treatment of oral candidiasis (thrush)
Treatment of cutaneous candidiasis
Treatment of gastrointestinal candidiasis

Cat (Feline)

Treatment of oral candidiasis
Treatment of cutaneous candidiasis
Treatment of gastrointestinal candidiasis

Horse (Equine)

Treatment of oral candidiasis
Treatment of gastric candidiasis (e.g., in foals)

Cattle (Bovine)

Treatment of oral candidiasis
Treatment of mycotic rumenitis (in calves)

Small Ruminants (Sheep / Goat)

Treatment of oral candidiasis
Treatment of gastrointestinal candidiasis

Rabbit & Small Mammals

Treatment of oral candidiasis
Treatment of gastrointestinal candidiasis

Avian & Poultry

Treatment of crop candidiasis (thrush) in poultry and pet birds

Exotic & Other Species

Treatment of candidiasis in reptiles (oral or topical)
Treatment of candidiasis in small mammals (e.g., ferrets, guinea pigs)

Pharmacology & Mechanism of Action

Drug Class: Antifungal | Pharmacological Group: Polyene macrolide antibiotic

Mechanism of Action: Nystatin binds to ergosterol, a major component of the fungal cell membrane, forming pores that increase membrane permeability. This leads to leakage of intracellular ions (especially potassium) and small molecules, causing cell death. It is fungistatic or fungicidal depending on concentration, and is effective primarily against Candida species and other yeasts.

Pharmacodynamics: Nystatin is a polyene antifungal with activity against a wide range of yeasts and fungi, including Candida albicans, Cryptococcus neoformans, Histoplasma capsulatum, and Aspergillus species. It is not absorbed from the gastrointestinal tract, so systemic effects are minimal. Its action is dose-dependent, with higher concentrations being fungicidal. Resistance is rare but can develop with prolonged use.

⚡ Pharmacokinetics Summary

Absorption: Nystatin is poorly absorbed from the gastrointestinal tract, with less than 5% absorbed after oral administration. It is also not absorbed through intact skin or mucous membranes. Topical application results in local action only.

Distribution: Due to poor absorption, systemic distribution is negligible. When applied topically, it remains at the site of application. In the GI tract, it acts locally on the lumen.

Metabolism: Nystatin is not significantly metabolized; it is largely excreted unchanged in the feces.

Excretion: The majority of an oral dose is excreted unchanged in the feces. Any absorbed fraction is excreted in the urine and bile.

Half-Life: Not well-defined due to lack of systemic absorption; local half-life is not clinically relevant.

Bioavailability: Oral bioavailability is negligible (<5%).

Protein Binding: Not applicable due to lack of systemic absorption.

Available Formulations & Strengths

Oral Suspension 100,000 units/mL (PO)

Oral Tablet 500,000 units (PO)

Oral Tablet 1,000,000 units (PO)

Topical Cream/Ointment 100,000 units/g (Topical)

Topical Powder 100,000 units/g (Topical)

Contraindications & Clinical Warnings

Contraindications:

Hypersensitivity to nystatin or any component of the formulation
Systemic fungal infections (nystatin is not absorbed systemically)
Concurrent use with other polyene antifungals (e.g., amphotericin B) may be antagonistic

Warnings & Clinical Precautions:

Use with caution in animals with gastrointestinal motility disorders
Not effective against dermatophytes (e.g., Microsporum, Trichophyton)
For topical use, avoid contact with eyes
In food animals, observe withdrawal times
May cause gastrointestinal upset in some animals
Use only for the indication and species for which it is prescribed

Adverse Effects & Reactions

Common:

Gastrointestinal upset (nausea, vomiting, diarrhea) when given orally
Local irritation at application site

Serious / Severe:

Allergic reactions (rare)
Systemic toxicity is unlikely due to poor absorption

Rare:

Contact dermatitis
Stevens-Johnson syndrome (very rare)

Key Drug Interactions

Interacting Drug / Class	Clinical Effect & Mechanism	Severity
Amphotericin B	Potential antagonism; nystatin may reduce the efficacy of amphotericin B if used concurrently.	Moderate
Oral anticoagulants (e.g., warfarin)	Nystatin may alter gut flora, potentially affecting vitamin K synthesis and increasing anticoagulant effect.	Mild
Immunosuppressive agents	May increase risk of fungal infections, but no direct interaction.	Mild

Overdose & Toxicity Management

Signs of Toxicity:

Gastrointestinal upset (vomiting, diarrhea)
In rare cases, systemic toxicity if massive doses are ingested (unlikely due to poor absorption)

Emergency Treatment Protocol: Treatment is symptomatic and supportive. Induce vomiting if recent ingestion and large amounts, but due to low toxicity, supportive care is usually sufficient. Monitor for dehydration and electrolyte imbalances.

Food Animal Withdrawal Times

🥩 Meat: 0 days🥛 Milk: 0 days🥚 Eggs: 0 days

Nystatin is not absorbed systemically, so withdrawal times are typically zero. However, always follow local regulations and label instructions.

Storage, Handling & Regulatory Information

Storage Temperature: Store at controlled 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. Do not freeze oral suspension.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Approved for veterinary use in dogs, cats, and poultry; also available as an OTC for topical use in some formulations.

Extra-Label (Off-Label) Use: In the US, nystatin is approved for use in dogs, cats, and poultry. Extra-label use in other species is permitted under AMDUCA with appropriate veterinary oversight and withdrawal times.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Nystatin is a first-line treatment for localized candidal infections, particularly in the oral cavity and gastrointestinal tract. It is not absorbed systemically, making it safe for use in pregnant and lactating animals. For systemic fungal infections, other antifungals such as fluconazole or itraconazole are preferred. When treating oral candidiasis, it is important to address underlying causes such as immunosuppression, antibiotic therapy, or poor nutrition. Topical formulations are useful for skin and mucosal lesions. In food animals, withdrawal times are generally zero, but always check label and local regulations.

References & Literature

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