Just wanted to add that I was prescribed Bactrim in 2009 for a Blastocystis infection. Trusting the doctor, I took it and had a year and a half of hell afterwards. According to lab tests, it got rid of the Blasto, but I had a severe reaction of C Dif - yellow, watery diarrhea - (nothing but yellow water) coming out of me for a month, chronic, every day and it completely terrified me. To top it all off, I couldn't get any doctor to help with this and it took very high doses of probiotics to restore my GI system to normal. I took Flagyl initially and it worked (for a while) but did not cure the problem so probably better not to take it and just stick to 200 to 400 billion units of a good quality probiotic daily instead.