small molecule

A small molecule is the classic, pill-sized drug: a compact chemical, far tinier than a protein, that is small enough to slip through cell membranes and reach targets deep inside cells. Aspirin, caffeine, and most familiar tablets are small molecules.

Technically, it is a low-molecular-weight organic compound, typically the classic synthetic drug type, usually weighing well under about 900 daltons. Its small size and tunable structure let chemists fine-tune how tightly it binds, how it dissolves, and how the body handles it.

Small molecules are usually cheap to manufacture, stable, and often work when swallowed, which is why they dominate oral medicines. Their main limitation is that some targets, such as large flat protein surfaces, offer no good pocket for a small molecule to grip, which is partly why other modalities exist.