DEAR DR. ROACH: After a month of testing from my gastrointestinal doctor, I have been diagnosed with irritable bowel syndrome (IBS). I have been sick for several months and have lost 12 pounds. (I’m now down to 100 pounds.)

I have been prescribed Xifaxan. The cost from my pharmacy is $811. Normally, I never have a co-pay. I checked around, and there is no discount with any other pharmacies. I read many chats about the medicine, and 50% of people in these chats believe that it does not work well and is very hard to digest for the two weeks that you have to take it.

I am meeting with a nutritionist to increase my weight, but every couple of days, I get so sick due to the IBS and can’t gain weight. What are your thoughts on taking this medication? –

–V.H.

ANSWER: IBS is a functional disease of the intestine. The cause is not certain, and there are several factors involved in IBS, including abnormal movement of the intestine; changes in pain sensitivity; unhealthy bacteria in the gut; and often sensitivities to certain foods. The hallmark of IBS are changes in bowel habits (diarrhea, constipation or both) and abdominal pain, which is often but not always improved after a bowel movement.

Xifaxan (rifaximin) is an antibiotic that is not well-absorbed in the body, so it only kills the bacteria in the gut. It is intended to preferentially kill harmful bacteria in the gut, improving the microbiome and decreasing bacterial products that cause symptoms of bloating and diarrhea. I have several patients who take it occasionally and their experience has been favorable.

When used wisely, it can help people with their symptoms. Of course, patients in whom it doesn’t work aren’t going to use it again, but it does work well for some people. The $811 cost with insurance is much better than the $2,500 to $3,200 cost for a course of treatment without insurance. People do not take it for a long time; my patients typically take a course one to four times per year. If it helps you, I hope you will find it worth the expense, but I don’t know whether you are fortunate to have this much money to spend on prescriptions over and above your insurance costs.

Severe weight loss with IBS is not common and should be considered an alarming symptom. I have seen many patients who were diagnosed with IBS, sometimes for years, before another comprehensive evaluation is performed, sometimes leading to the diagnosis of inflammatory bowel disease (Crohn’s disease or ulcerative colitis). If you have not had a recent evaluation, including an endoscopy, it may be time to repeat these.

ToYourGoodHealth@med.cornell.edu.