Griseofulvin is an antifungal antibiotic derived from a species of Penicillium. Griseofulvin is used in the treatment of fungal infections of the skin, hair, and claws and ringworm. It is useful in treatment of infections caused by Trichophyton mentagrophytes, T. rubrum, T. schoenleini, T. sulphurem, T. verrucosum, T. interdigitale, Epidermophyton floccosum, Microsporum gypseum, M. canis, and M. audouini in dogs and cats. Griseofulvin is a fungistatic. Following oral administration, griseofulvin is deposited in the keratin precursor cells and has a greater affinity for diseased tissue. The drug is tightly bound to the new keratin which becomes highly resistant to fungal invasions. Only new hair and nail growth is resistant to infection. Griseofulvin is not active against bacteria or other pathogenic fungi. Griseofulvin is used in horses for treating ringworm infection caused by Trichophyton equinum. Adverse affects are uncommon at usual doses. Cats, particularly kittens, may be more susceptible to adverse effects. This may be due to the fact that cats metabolize this drug at a slower rate than dogs and humans.