what actually happens when a molecule activates a receptor vs when an inhibitor molecule just blocks it?

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How is it that two different molecules can fit into a slot when one causes something to happen and the other just sits there doing nothing?

Extra question: what mechanism cleans up the inhibitor? Let’s say with an SSRI? 

In:

Think of the molecule as a key and the receptor as a lock. The “real” key will always be able to open the lock as intended. However, there are some keys that coincidentally have a similar shape to the “real” key, but not similar enough to open the lock. So when you try to open the lock with the similar key, instead of opening the lock, it gets jammed and it gets stuck. The lock is still locked while the key is stuck, which is a metaphor for how the inhibitor just gets stuck in the receptor without causing a biological effect.

Regarding your extra question, most drugs are carried in the blood to the liver where they are broken down or modified to form more water soluble products that can be easily removed in the urine. The reason SSRIs can’t be taken with some medication is because they inhibit certain liver enzymes. This causes drugs that are normally modified by the enzyme to remain in the body for longer and at higher concentrations which can cause toxic effects.