If you have diarrhea and related symptoms, the cause may be difficult to diagnose. Even if blastocystis is found in your stool, it might not be causing your symptoms. Often another food- or water-borne organism is the likely cause of illness.

Your doctor will take your medical history, ask you about recent activities, such as traveling, and perform a physical exam. A number of lab tests help diagnose parasitic diseases and other noninfectious causes of gastrointestinal symptoms:

  • Stool (fecal) exam. This test looks for parasites. Your doctor might give you a container with preservative fluid for your stool sample. Refrigerate — don't freeze — your samples until you take them to your doctor's office or lab.
  • Endoscopy. If you have symptoms, but the fecal exam doesn't reveal the cause, your doctor might request this test. When you're under sedation, a special camera on a tube is used to exam your digestive tract.
  • Blood tests. A blood test that can detect blastocystis is available but not commonly used. However, your doctor might order blood tests to look for other causes of your signs and symptoms.

More Information


If you have a blastocystis infection without signs or symptoms, then you don't need treatment. Mild signs and symptoms might improve on their own within a few days.

Potential medications for eliminating a blastocystis infection and improving symptoms include:

  • Antibiotics, such as metronidazole (Flagyl) or tinidazole (Tindamax)
  • Combination medications, such as sulfamethoxazole and trimethoprim (Bactrim, Septra, others)
  • Anti-protozoal medications, such as paromomycin or nitazoxanide (Alinia)

Responses to these medications vary greatly. Also, because the organism might not be the cause of your symptoms, improvement might be due to the medication's effect on another organism.

Preparing for your appointment

You'll likely see your primary care doctor. However, in some cases, you might be referred to someone who specializes in either infectious disease or in digestive system disorders (gastroenterologist).

Here's some information to help you get ready for your appointment.

What you can do

Be aware of pre-appointment restrictions. When you make the appointment, ask if there's anything you need to do in advance, such as restrict your diet.

Make a list of:

  • Your symptoms, and when they began
  • Key personal information, including any major stresses or recent life changes and whether you've recently traveled to a developing country
  • All medications, vitamins or supplements you take, including doses
  • Questions to ask your doctor

Questions to ask your doctor include:

  • What's the most likely cause of my symptoms?
  • Are there other possible causes?
  • What tests do I need?
  • What treatments are available, and which one do you recommend for me?
  • Should I change my diet?
  • Are there brochures or other printed material that I can take home with me? What websites do you recommend?

Don't hesitate to ask other questions.

What to expect from your doctor

Your doctor is likely to ask you questions, including:

  • Do you have symptoms all the time, or do they come and go?
  • How severe are your symptoms?
  • Does anything seem to improve your symptoms?
  • What, if anything, appears to worsen your symptoms?
  • Do you have any other health conditions?

What you can do in the meantime

If your symptoms are related to blastocystis, they'll likely go away on their own before you even see your doctor. Stay well hydrated. Oral rehydration solutions — available through drugstores and health agencies worldwide — can replace lost fluids and electrolytes.

Feb. 09, 2023
  1. Jameson JL, et al., eds. Protozoal infections and trichomoniasis. In: Harrison's Principles of Internal Medicine. 20th ed. New York, N.Y.: The McGraw-Hill Companies; 2018. https://accessmedicine.mhmedical.com. Accessed Dec. 2, 2020.
  2. Blastocystis spp. FAQs. Centers for Disease Control and Prevention. https://www.cdc.gov/parasites/blastocystis/faqs.html. Accessed Dec. 2, 2020.
  3. Leder K, et al. Blastocystis species. https://www.uptodate.com/contents/search. Accessed Dec. 2, 2020.
  4. Diarrhea. Merck Manual Professional Version. https://www.merckmanuals.com/professional/gastrointestinal-disorders/symptoms-of-gastrointestinal-disorders/diarrhea. Accessed Dec. 3, 2020.
  5. When and how to wash your hands. Centers for Disease Control and Prevention. https://www.cdc.gov/handwashing/when-how-handwashing.html. Accessed Dec. 2, 2020.
  6. Food and water safety. Centers for Disease Control and Prevention. https://wwwnc.cdc.gov/travel/page/food-water-safety. Accessed Dec. 2, 2020.
  7. Water disinfection. Centers for Disease Control and Prevention. https://wwwnc.cdc.gov/travel/page/water-disinfection. Accessed Dec. 3, 2020.
  8. Freedman S. Oral rehydration therapy. https://www.uptodate.com/contents/search. Accessed Dec. 2, 2020.


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