DEAR DR. ROACH: I am a 76-year-old male who is in relatively good health. For the past six months, I have been experiencing diarrhea on and off. I initially spoke to my general physician, and he thought it could be lactose intolerance. I reduced my dairy intake, drank lactose-free milk, and took Lactaid pills before eating dairy.

After about three to four months, this did not seem to help, and I got worse. I then saw my gastroenterologist, and he did blood tests and a gastrointestinal pathogen panel. I tested negative for celiac disease, but my general physician said that Clostridioides difficile (C. diff) and rotavirus A were detected.

My gastroenterologist put me on oral vancomycin four times a day for 10 days. This did not work, so he put me on a longer course for another month. I am still not better. It seems like I am in a pattern of feeling OK with small regular stools for three days, then get diarrhea for two days with very loose, not quite watery stools. Then the pattern starts all over again — three good days, then two bad days. I do not have any pain or nausea.

What do you think this could be, and what can I do to get over this?

— J.V.F.

ANSWER: C. diff is an uncommon but quite serious cause of diarrhea, and it is often associated with antibiotics. (Amoxicillin is the most common, while clindamycin is the “classic” cause, but almost all antibiotics can be associated with the development of C. diff.) It is also associated with hospital stays. It can happen spontaneously in the community as well, which seems to be the case with you.

C. diff is notoriously difficult to treat. It’s likely that you failed the treatment with vancomycin, but this should be confirmed with another stool test, as there are many other possibilities. The GI pathogen panel can sometimes produce a false positive, so a dedicated C. diff test should be done before proceeding if it hasn’t already been done.

Vancomycin has been one of the most effective treatments for C. diff, but still, initial failure rates are 10-15%. Thirty-day success rates are as low as 56% with vancomycin. Because of this, expert groups have changed the recommendation for treatment to fidaxomicin (Dificid), which has a 30-day success rate of 87%.

Since vancomycin did not work for you, fidaxomicin would be the next recommended regimen. Unfortunately, it is quite expensive, with a 10-day course of treatment costing somewhere around $4,000 in United States retail pharmacies. (Prescription savings programs like GoodRx can make this as low as $1,300.) Some insurance companies won’t pay for fidaxomicin unless a person has failed with vancomycin, as you have.

Readers may email questions to ToYourGoodHealth@med.cornell.edu.