Chemistry - A fugistatic antibiotic produced by species of Penicillium (primarily P. griseofulvum), griseofulvin occurs as an odorless or nearly odorless, bitter tasting, white to creamy white powder. It is very slightly soluble in water and sparingly soluble in alcohol.
Two forms of the drug are available commercially. Microsize griseofulvin contains particles with a predominant size of 4 µm in diameter, while the ultramicrosize form particle size averages less than 1 micron in diameter.
Storage/Stability/Compatibility - Although griseofulvin is relatively thermostable, products should be stored at less than 40°C, preferably at 15-30°C. Griseofulvin suspension should be stored in tight, light-resistant containers. Microsize tablets and capsules should be stored in tight containers; the ultramicrosize tablets should be stored in well-closed containers.
Pharmacology - Griseofulvin acts on susceptible fungi by disrupting the structure of the cell’s mitotic spindle, thereby arresting the metaphase of cell division. Griseofulvin has activity against species of Trichophyton, Microsporum and Epidermophyton. Only new hair or nail growth is resistant to infection. It has no antibacterial activity, and is not clinically useful against other pathogenic fungi.
Uses/Indications - In veterinary species, griseofulvin is approved for use in dogs and cats to treat dermatophytic fungal (see above) infections of the skin, hair and claws, and to treat ringworm (caused by T. equinum and M. gypseum) in horses. It has also been used in laboratory animals and ruminants for the same indications.
Pharmacokinetics - The microsized form of the drug is absorbed variably (25-70%); dietary fat will enhance absorption. The ultramicrosize form of the drug may be nearly 100% absorbed. Generally, the ultramicrosize form is absorbed 1.5 times as well as the microsized form for a given patient.
Griseofulvin is concentrated in skin, hair, nails, fat, skeletal muscle and the liver, and can be found in the stratum corneum within 4 hours of dosing.
Griseofulvin is metabolized by the liver via oxidative demethylation and glucuronidation to 6-desmethylgriseofulvin which is not active. In humans, the half-life is 9-24 hours. A serum half-life of 47 minutes has been reported for dogs. Less than 1% of the drug is excreted unchanged in the urine.
Contraindications/Precautions/Reproductive Safety - Griseofulvin is contraindicated in patients hypersensitive to it or with hepatocellular failure.
Because kittens may be overly sensitive to the adverse effects associated with griseofulvin, they should be monitored carefully if treatment is instituted.
Griseofulvin is a known teratogen in cats. Dosages of 35 mg/kg given to cats during the first trimester caused cleft palate, and other skeletal and brain malformations in kittens. Griseofulvin may also inhibit spermatogenesis. Because dermatophytic infections are not generally life-threatening and alternative therapies are available, use of the drug should be considered contraindicated during pregnancy.
Adverse Effects/Warnings - Griseofulvin can cause anorexia, vomiting, diarrhea, anemia, neutropenia, leukopenia, depression, ataxia, hepatotoxicity or dermatitis/photosensitivity. With the exception of GI symptoms, adverse effects are uncommon at usual doses. Cats, particularly kittens, may be more susceptible to adverse effects than other species. This could be due to this species’ propensity to more slowly form glucuronide conjugates and thus metabolize the drug at a slower rate than either dogs or humans.
Overdosage/Acute Toxicity - No specifics regarding griseofulvin overdosage or acute toxicity was located. It is suggested that significant overdoses be handled with gut emptying, charcoal and cathartic administration unless contraindicated. Contact a poison control center for more information. Horses have received 100 mg/kg PO for 20 days without apparent ill effect.
Drug Interactions - Phenobarbital (and other barbiturates) has been implicated in causing decreased griseofulvin blood concentrations, presumably by inducing hepatic microsomal enzymes and/or reducing absorption. If phenobarbital and griseofulvin are given concurrently, griseofulvin dosage adjustment may be necessary. Coumarin (e.g., warfarin) anticoagulants may have their anticoagulant activity reduced by griseofulvin; anticoagulant adjustment may be required. Griseofulvin may potentiate the effects of alcohol.
Note: all doses are for microsize preparations unless otherwise indicated.
For susceptible dermatophytic infections:
a) 10 mg/kg PO once daily (Robinson 1987)
b) 10 mg/kg PO (in feed) daily for 7 days (Brumbaugh 1987)
Monitoring Parameters -
1) Clinical efficacy; culture
2) Adverse effects
3) CBC q2-3 weeks during therapy
4) Liver enzymes (if indicated)
Client Information - Clients should be instructed in procedures used to prevent reinfection (destruction of old bedding, disinfection, periodic reexaminations, hair clipping, etc.) and the importance of compliance with the dosage regimen. Should animal develop adverse effects other than mild GI disturbances, they should contact their veterinarian.
Dosage Forms/Preparations/FDA Approval Status/Withholding Times -
Griseofulvin (Microsize) Powder 2.5 g griseofulvin in 15 g sachets
Fulvicin-U/F® Powder (Schering-Plough); (Rx) Approved for use in horses.
Griseofulvin (Microsize) Tablets 250 mg, 500 mg (scored); Fulvicin-U/F® Tablets (Schering-Plough); (Rx) Approved for use in dogs and cats.
Griseofulvin (Microsize) Capsules 250 mg; Grisactin ® (Wyeth-Ayerst); (Rx)
Griseofulvin (Microsize) Tablets 250 mg, 500 mg; Fulvicin-U/F® (Schering); Grifulvin V® (Ortho); Grisactin 500® (Wyeth-Ayerst); (Rx)
Griseofulvin (Microsize) Oral Suspension 125 mg/5 ml in 120 ml; Grifulvin V® (Ortho Derm); (Rx)
Griseofulvin (Ultramicrosize) Tablets 125 mg, 165 mg, 250 mg, 330 mg; Fulvicin P/G® (Schering); Grisactin Ultra ® (Wyeth-Ayerst); Gris-PEG® (Allergan Herbert); generic. (Rx)
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