Sulfadiazine is an antibiotic called sulfanilamide.Although sulfadiazine antibiotics are now rarely prescribed, sulfadiazine is still a useful drug to help prevent the recurrence of rheumatic fever, which is caused by a bacterial infection called streptococcus and can cause serious damage to the heart.There is a risk of further illness after the first episode, so long-term use of antibiotics such as sulfadiazine is needed to help prevent it.Sulfadiazine is occasionally prescribed for other reasons.