Is it true that 50% of what we think about a substance now will be discredited in 15 years time?

I think that comes from people's misunderstanding of science, and how it works. We have scientific consensuses based on the available evidence. New evidence can come to light, and frequently does, that changes that consensus. It's not that we know X does this for sure. It's that a specific study design showed X effect in a certain situation and animal/human model. So don't think of it like 50% of what we know for sure to be true will eventually become false. Think of it as we are constantly evolving our ability to observe how things work, and are constantly updating the consensus on compounds. When you think of it that way, it's not that we are wrong most of the time. It's that we are constantly improving our understanding of human cognition, and how exogenous compounds interact with it. That's a much more constructive way of looking at it. I'm not a huge fan of Tim Ferriss, but he was probably trying to simplify that concept for people listening to Joe Rogan.