Pyoderma gangrenosum (pie-oh-DER-muh gang-ruh-NO-sum) is a rare condition that causes large, painful sores (ulcers) to develop on your skin, most often on your legs.
It's not certain what causes pyoderma gangrenosum, but it appears to be a disorder of the immune system. People who have certain underlying conditions, such as inflammatory bowel disease or rheumatoid arthritis, are at higher risk of pyoderma gangrenosum.
Treatment typically includes high doses of corticosteroids, such as prednisone, along with other drugs designed to suppress your immune system. It can take weeks or even months to heal the ulcers associated with pyoderma gangrenosum.