Something I see a lot these days, what with modern scientism and all, is that even a tiny amount of $x causes permanent damage. You hear it about everything, from sunlight exposure to alcohol to sleep deprivation to red meat consumption. Most of the studies, of course, are an amusing combination of poor study design, funding bias and/or p-hacking, but occasionally a well-designed independently funded study by honest people slips through. Typically these studies will show the effect size so small as to be negligible. For example the Nature study about alcohol consumption shrinking brains showed an effect size over a lifetime of drinking equivalent to a fraction of the brain shrinkage that occurs over a normal year. Statistically significant? Yes. Practically significant, and enough to outweigh any benefits? Ehh...

But the larger problem is that the slabs of thinking meat that we inhabit are the most complex system in the known universe, and it is usually impossible to understand the effects of a single chemical. I wrote about this here a few months ago: https://news.ycombinator.com/item?id=42326209