How does a drug find the right cells in a whole body?

A drug finds the right cells because those cells carry the right receptors. A receptor is a protein, usually on the surface of a cell, that receives a chemical message and turns it into a response, for example a change in the cell's electrical activity. The molecule that binds to it is called a ligand, and the ligand can be a hormone, a neurotransmitter, a toxin, or a pharmaceutical drug. Each receptor binds only ligands of a particular shape, which people compare to locks that accept only specifically shaped keys.

This is how a drug can act on some cells and not others: the body already uses the lock. The drug is a key that happens to fit, or a copy of one. The nicotinic acetylcholine receptor is a good example: the body's own key is acetylcholine, nicotine can also turn it, and curare can block it.

Fitting is only half of the story. Scientists separate how tightly a ligand binds, called affinity, from how well it switches the receptor on once bound, called efficacy. Some ligands bind and activate, and are called agonists. Others bind and do nothing, and by occupying the lock they stop other keys from working. Those are antagonists.

So a medicine can work in two opposite ways: by imitating a message, or by blocking one. In both cases the shape of the molecule decides what it can do, and that is why chemists spend so much effort adjusting shapes.