Diarrhea is common in patients with the human immunodeficiency virus (HIV). Infections with the organisms Isospora or Cyclospora frequently cause diarrhea in those patients. The antibiotic trimethoprim–sulfamethoxazole is effective treatment for these infections, usually given at a dose of one tablet four times per day for 10 days. It is unknown whether lower doses or shorter courses of this antibiotic would be effective. Since these infections often come back if the treatment is stopped, patients are usually continued on the antibiotic (secondary prophylaxis). Unfortunately, some patients develop side effects related to the trimethoprim–sulfamethoxazole, and others may be allergic to it. Another antibiotic, ciprofloxacin, has been suggested as an option for treating and preventing infections with Isospora and Cyclospora species.