Action - Reaction

I've heard it said that every action has a reaction. I don't understand what that means.

 Every action has a reaction. That means that when you take a drug, any drug, including caffeine, each is designed to produce a reaction. The reaction depends on the specific drug and what it does when it binds to receptor molecules on the surface of the cell and gets inside. Think of the drug as a key and the receptor molecule as a lock. The drug must find a lock that fits its key. Once inside, the drugs usually alter the cell in some way, release a brain chemical that will give you a reaction. Some drugs make you sleepy, some wake you up, some trigger the Brain Reward System to release dopamine to make you feel better. Some can trigger a mental disorder such as a psychotic episode, and some can eventually kill you. Just remember when you make choices, that every action has a reaction.