That is a great question and one I have gone over many times in my head. I came to the conclusion that it was either one of the two things you suggested - either placebo, or maybe I very briefly entered a sweet spot. Another thought I've had is that the basis for my "feeling great" comment was based on having a high energy day in the gym shortly after taking the AI. But those days come and go seemingly at random so maybe it had nothing to do with the AI.
A few things I can say for sure were not placebo were some of the negative effects. Mentally, I was truly expecting to have better libido/erectile function - the opposite occurred. I had heard about people experiencing joint pain, etc from AIs - I was aware of that side effect but did not experience it. Additionally, I had never heard of anyone saying that an AI made them feel dumb mentally or feel slightly depressed - and those for me were the worst (and most surprising) side effects.
