Post on MDMA. Multiple people have told me it convinced them not to use MDMA, or allowed them to convince others not to do so. Note that this post is 7 years old, and if I was doing it today it would be much more quantified
This is true for me! I have had a bunch of friends over the years who considered doing MDMA, and the previous post was quite helpful for changing those people's minds on what drugs to take (which I think was in-expectation quite good for them). My guess is it prevented about 1.5 people in-expectation from doing MDMA at some point as a result of me linking to it.