Probiotics are used by a large number of people. I know there is research on their efficacy, but have any studied reached what would reasonably be considered a phase III clinical trial?
You can search on the clinical trials website to find most of the trials conducted with probiotics.
https://clinicaltrials.gov/ct2/results?intr=%22Lactobacillus%22 gives currently 589 results, some of which are described as phase 3 studies. You can change the search term to
acidophilus or whatever you're interested in.