I have been doing some self-experimentation. One thing I did was to try to find the best dose of caffeine for me. I tried regular coffee, decaff, and coffee and caffeine pills; several times and in different orders. With as close to controlled conditions as one person alone could arrange, I discovered that caffeine has absolutely no effect on me. I could discern absolutely no difference in alertness, learning (read a textbook chapter and did test at end), or reaction time (simple video game). I already knew I didn't react strongly to caffeine, hence the experimenting, but the result was a surprise. Apparently all of the effects I had previously attributed to caffeine had been placebo.
I had already known that I got little or no benefit from acetaminophen (Tylenol). The little relief I did get was easy to attribute to placebo effect.
I have begun wondering how much of the lower than unity effectiveness of most drugs and medications is the result of others who are totally unaffected by some particular drug. Rather than people who are just less affected.