system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can such a small dose of medication have such a huge impact in the way our bodies work?

I take some medications prescribed by my doctor. These medications are all prescribed in milligram doses ... when you take into consideration I’m a male of 190lbs, I just can’t fathom that such small quantities can have such drastic effects on how my body functions. Another example: I hear on the news the rise of drugs laced with Fentanyl and that all it takes is a few grains to kill you. How is this even possible?
gpt
This question is wildly broad, since the mechanism of action for various substances can be wildly different. But metaphorically, it's the same way you can demolish a high rise without explosives on every wall. We think of our bodies as a distinct unit, but they aren't. They're huge conglomerations of various systems. You don't need to affect everything; you just need to affect one special piece of a given system.