It's tempting to think of a drug as something that simply floods the body and produces an effect in proportion to the dose. Pharmacodynamics — the study of what a drug does to the body, as opposed to pharmacokinetics, the study of what the body does to the drug — describes something more specific: most drugs work by binding to particular receptors, molecular structures on or in cells shaped to interact with specific chemical messages, and a drug's entire profile of effects follows from exactly which receptors it fits and how tightly.
Receptors work like locks, and drugs like keys of varying quality
A receptor is a protein structure, usually on a cell's surface, shaped to bind a specific molecule — normally the body's own signalling chemicals, like hormones or neurotransmitters — and trigger some downstream effect inside the cell once that binding happens. A drug can be designed to bind the same receptor. An agonist is a drug that binds a receptor and activates it, triggering the same kind of downstream effect the body's natural signalling molecule would. An antagonist binds the receptor but doesn't activate it — instead, it occupies the site and physically blocks the natural molecule, or an agonist drug, from binding there at all, effectively silencing that receptor's signal rather than triggering it. Both kinds of drug depend entirely on the shape-based fit between the drug molecule and the receptor; a drug that doesn't fit a given receptor simply has no effect through that pathway, no matter how much of it is present.
Why the same dose can mean wildly different things
This is why a drug's potency and its side-effect profile trace back to receptor binding rather than to raw dose alone. A drug that binds its intended receptor very tightly and selectively can be effective at a tiny dose, while a less selective drug might need a much larger dose to reach the same effect — and if that larger dose also happens to bind other, unrelated receptors elsewhere in the body, those unintended interactions show up as side effects. Two drugs can share the identical intended effect through completely different receptor mechanisms, with very different dosing requirements and very different side-effect profiles, purely because of how selectively and how tightly each one's molecular shape actually matches its target.
What we're still unsure about
The lock-and-key model of receptor binding is well established and forms the foundation of modern drug design, but it's a simplification of what's often a more dynamic reality — many receptors can shift between multiple shapes, bind partially or with varying efficiency, and interact with more than one downstream signalling pathway depending on subtle differences in how a drug binds them. Predicting exactly how a newly designed molecule will bind a target receptor, and whether it will also bind unintended receptors elsewhere, still isn't fully solvable from chemical structure alone, which is a major reason drug development still requires extensive laboratory and clinical testing rather than being achievable through computation and receptor modelling alone.
This sits inside Pharmacodynamics & Receptor Theory, one of seven topics in Pharmacology, one of four domains in Medicine, one of seventeen subjects the app can quiz you on.