Are you familiar with c diff? Placebo's don't work. It's dangerous to have placebo controlled trials with this disease. The current standard of care is a course of very strong antibiotics which still leave > 25% chance of recurrence. These antibiotics are most often used as a control group in c diff infections.

In general, placebo controlled trials are usually conducted for diseases which don't have an effective standard of care, or for conditions which aren't serious enough to be considered life threatening. For more serious diseases where a standard of care exists (standard of care is usually scientifically discovered but validated by placebo controlled trials), you conduct trials and compare the performance of the new drug/therapy with the existing standard of care as the control group.

Also the result you care about is often not "does it work?" but "does it work better than the current approach?".

Yup. It's like wanting placebo controlled trials for cancer. It would be unethical and ungodly harmful to condemn people to suffering and death just so people could gather data that wouldn't even compare the results to the current best practices.

>Are you familiar with c diff? Placebo's don't work

More familiar than you apparently, because placebos have been proven to work:

https://www.medscape.com/viewarticle/853175

I think the problem is the subjective measure, so it's impossible to know which symptoms are caused by c.diff and which from IBS.

"The benefits, risks, burdens, and effectiveness of the new methods should be tested against those of the best current prophylactic, diagnostic, and therapeutic methods. This does not exclude the use of placebo or no treatment in studies where no proven prophylactic, or therapeutic methods exist." World Medical Association (WMA) Declaration of Helsinki.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2647647/ for more.

Placebo controlled trials for C Diff would be unethical thus would likely not be legal in any country that conduct their trials with any ethical standards. Just like you wouldn't have a placebo group in a trial for an new AIDS or cancer treatment. In this case we're looking for a treatment that works just as well or better than current treatments.

Well clearly that's not true because placebo controlled trials into FMT for c.diff have been done in the USA. AIDS and cancer have objective measures, whereas with c.diff the trials just look at subjective measures.

Also, given that the placebo improvement rates are 40-90% in all c.diff trials, it would be unethical to not use a placebo arm.