Nitazoxanide is used to treat diarrhea in adults and children older than 1 year of age caused by the protozoa Cryptosporidium or Giardia. Protozoa are suspected as the cause when diarrhea lasts more than 7 days. Nitazoxanide is in a class of medications called antiprotozoal agents. It works by stopping the growth of certain protozoa that cause diarrhea.