griseofulvin, drug produced by the molds Penicillium griseofulvum and P. janczewski and used in the treatment of ringworm, including athlete’s foot and infections of the scalp and nails. Griseofulvin exerts its antimicrobial activity by binding to microtubules, cellular structures responsible for forming mitotic spindles during cell division and for processing cell wall components needed for growth. A common side effect is headache.