Drug Monograph
Full clinical overview, indications, dosage references & safety notes.
Overview
Griseofulvin (Fulvicin®, Gris-PEG®) is a fungistatic antibiotic used in dogs and cats for the treatment of dermatophytosis affecting the skin, hair, and claws. Its use in small animal practice has declined significantly due to the availability of safer and more effective antifungal agents such as azoles and terbinafine.
The drug is active only against dermatophytes (e.g., Microsporum, Trichophyton, and Epidermophyton) and has no clinically useful activity against other fungi such as Malassezia. Clinical response depends on the rate of keratin turnover, as only newly formed hair and keratin become resistant to infection during therapy.
Mechanism of Action (MOA): Griseofulvin interferes with fungal cell division by disrupting the mitotic spindle and inhibiting nucleic acid synthesis. It is incorporated into keratin precursor cells, protecting newly formed keratin from fungal invasion rather than directly eliminating existing infection.
Griseofulvin is poorly water-soluble and variably absorbed from the gastrointestinal tract, with improved absorption when administered with fatty meals. The ultramicrosize formulation has significantly better bioavailability than the microsize form. The drug is distributed to keratinized tissues such as skin, hair, and nails and undergoes hepatic metabolism.
Indications
Griseofulvin is indicated in dogs and cats for the treatment of dermatophytic fungal infections involving keratinized tissues. It is primarily used when systemic therapy is required, although its use has declined due to the availability of safer and more effective antifungal agents.
- Dermatophytosis (ringworm): Treatment of infections caused by Microsporum and Trichophyton species affecting the skin, hair, and claws.
- Generalized or severe infections: Indicated when lesions are widespread, recurrent, or not responsive to topical therapy alone.
- Adjunct to multimodal management: Used alongside topical antifungal therapy and environmental decontamination to reduce reinfection and improve treatment success.
- Keratin-associated infections: Particularly useful in infections involving hair follicles and nails where drug incorporation into new keratin is beneficial.
Dosage (Reference)
Dog
In dogs, dosing varies significantly depending on whether the microsize or ultramicrosize formulation is used. Administration with a fatty meal is essential to improve absorption and therapeutic effectiveness.
| Clinical use | Route | Dose | Notes |
|---|---|---|---|
| Dermatophytosis (microsize) | PO | 50 mg/kg/day | Give once daily or divided BID; must be given with fatty food. |
| Dermatophytosis (ultramicrosize) | PO | 5–10 mg/kg/day | Better absorption → lower dose required. |
| Label dose (weight-based) | PO | 62.5–500 mg/dog | Based on body weight ranges: • ≤2.7 kg: 62.5 mg • 2.7–8.2 kg: 125 mg • 8.2–16.4 kg: 250 mg • 16.4–21.8 kg: 375 mg • 21.8–34.1 kg: 500 mg |
| Weekly regimen | PO | 22 mg/kg every 7–10 days | Dose = 22 mg/kg × number of days between doses; adjust to response. |
• Microsize and ultramicrosize formulations are NOT interchangeable.
• Always give with fatty food (e.g., cheese, cream) to improve absorption.
• Continue treatment for at least 2 weeks after clinical cure and after obtaining negative fungal cultures.
• Treatment duration may extend for several weeks to months.
• Adjust dose based on clinical response and tolerability.
Cat
In cats, griseofulvin should be used cautiously due to increased susceptibility to adverse effects, particularly bone marrow suppression. Lower-risk patients should be selected, and close monitoring is essential.
| Clinical use | Route | Dose | Notes |
|---|---|---|---|
| Dermatophytosis (microsize) | PO | 50 mg/kg/day | Give once daily or divided; administer with fatty meal. |
| Dermatophytosis (ultramicrosize) | PO | 5–10 mg/kg/day | Preferred due to improved absorption and reduced dosing. |
• Higher risk of bone marrow suppression—monitor CBC regularly (baseline and every 1–3 weeks).
• Avoid use in kittens unless clearly necessary due to increased sensitivity.
• Test for FIV before treatment due to increased risk of neutropenia.
• Always administer with fatty food to enhance absorption.
• Continue treatment for at least 2 weeks beyond clinical cure and until negative fungal cultures are confirmed.
Warnings & Precautions
Griseofulvin requires careful patient selection and monitoring due to its potential for serious adverse effects, particularly in cats and young animals.
- Pregnancy (contraindicated): Strong teratogen in dogs and cats; may cause severe congenital defects (e.g., cleft palate, skeletal and CNS abnormalities). Pregnancy status should be confirmed before use.
- Hepatic disease: Contraindicated in animals with hepatocellular failure; use cautiously in mild hepatic dysfunction due to hepatic metabolism of the drug.
- Bone marrow suppression: Risk of anemia, neutropenia, leukopenia, and thrombocytopenia, especially in cats.
- FIV-positive cats: Increased susceptibility to neutropenia and panleukopenia; testing is recommended before initiating therapy.
- Kittens: More sensitive to adverse effects, particularly bone marrow toxicity; require close monitoring if treated.
- Formulation differences: Microsize and ultramicrosize formulations are not interchangeable; incorrect substitution may result in underdosing or toxicity.
- Monitoring: Baseline and periodic CBC (every 1–3 weeks) are recommended during therapy; biochemical monitoring may also be indicated.
- Absorption considerations: Poor oral absorption unless given with fatty meals; inconsistent dosing may reduce treatment efficacy.
Drug Interactions
Griseofulvin can alter the metabolism and effectiveness of several drugs, primarily through hepatic enzyme induction and effects on drug clearance. Careful monitoring and dose adjustments may be required when used concurrently with the following medications.
- Phenobarbital: May decrease griseofulvin plasma concentrations, reducing antifungal efficacy.
- Cyclosporine: Griseofulvin may reduce cyclosporine concentrations, potentially decreasing immunosuppressive effectiveness.
- Warfarin: May reduce anticoagulant activity, increasing the risk of thrombosis.
- Estrogens and progestins (e.g., estriol, megestrol): May decrease hormone concentrations and effectiveness.
- Theophylline / aminophylline: May decrease half-life and plasma concentrations, reducing therapeutic effect.
- Aspirin (salicylates): May decrease salicylate concentrations.
- Alcohol (ethanol): May potentiate adverse effects when used concurrently.
Side Effects & Overdose
Side Effects
Adverse effects of griseofulvin are dose-dependent and occur more commonly in cats than in dogs, particularly in kittens and immunocompromised animals.
- Gastrointestinal: Anorexia, vomiting, diarrhea; relatively common and may limit compliance.
- Hematologic: Anemia, neutropenia, leukopenia, thrombocytopenia due to bone marrow suppression.
- Neurologic: Depression, lethargy, ataxia.
- Hepatic: Hepatotoxicity, especially with prolonged use.
- Dermatologic: Dermatitis and photosensitivity reactions.
- Species sensitivity: Cats (especially kittens) are more susceptible to bone marrow toxicity and adverse effects than dogs.
Overdose
Specific data on acute overdose in dogs and cats are limited. Toxicity is expected to exaggerate the known adverse effects, particularly CNS depression and bone marrow suppression.
- Clinical signs: Severe anorexia, vomiting, diarrhea, depression, and hematologic abnormalities.
- High-risk patients: Cats (especially FIV-positive) may develop fever, depression, diarrhea, and increased risk of neutropenia at high doses.
- Management: Supportive care including gastrointestinal decontamination (activated charcoal, cathartics) when appropriate.
- Monitoring: Close monitoring of CBC and clinical status is essential following suspected overdose.
Key Notes
Practical clinical points to optimize the use of griseofulvin in dogs and cats:
- Slow clinical response: Improvement depends on replacement of infected keratin, so visible recovery may take several weeks.
- Limited antifungal spectrum: Effective only against dermatophytes and not useful for other fungal infections.
- Palatability issues: Bitter taste may reduce compliance, especially in cats.
- Formulation awareness: Always confirm whether the product is microsize or ultramicrosize before prescribing.
- Declining clinical use: Often reserved for cases where newer antifungals are unavailable or unsuitable.
- Environmental control: Successful treatment requires strict hygiene and environmental decontamination to prevent reinfection.
