system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how does a painkiller like Percocet or Vicodin or tramadol work?
gpt
To first understand painkillers, you have to understand how pain works. Pain is a sensation, an electrical impulse sent from cell to cell to cell until it eventually reaches your brain and tells you "OW!" Each time the pain impulse passes through a cell, it has to cross the synapse, where it is converted into a chemical. This process is like hopping on a boat to cross a river; the impulse has to get off at a certain "dock" (receptors) which tells the next cell what to do with the signal. The next cell gets "pain" and the process repeats.

Painkillers like Vicodin work by binding to certain receptors in the synapse called Opioid receptors. These receptors essentially tell a cell that receives a pain signal to stop transmitting it. The impulse basically dies at that cell and, because your brain never receives it, you don't experience pain. 

Painkillers like Morphine and Vicodin are highly dangerous, however, because of the side-effects they have on the body. Your body has a natural painkiller called endorphins that it pumps out whenever it experiences pain. When the body gets that job taken care of by an outside source (ie, morphine), it stops producing endorphins. That's why opiates have such bad withdrawal symptoms. Your body is constantly producing endorphins naturally, so we are always experiencing a level of pain constantly (think about the kind of impulses your feet is sending when there is 150+ pounds of flesh and bone crushing them). When these endorphins aren't produced, you'll be in terrible pain along with feeling all sorts of things your body naturally blocks out; chills, tingles, sweats, etc. Opiate withdrawal is not a fun experience.