diffusion

Drop a little ink into still water and watch it spread out until the colour is even everywhere, with no stirring needed. That spreading is diffusion: the net drift of molecules from where they are crowded to where they are sparse, driven by nothing more than their constant random jiggling. Over time, random motion alone smooths out differences in concentration.

Diffusion is the engine behind much of how drugs move. A dissolved drug diffuses away from a dissolving tablet surface; a drug in a patch diffuses through skin; a drug inside a sustained-release matrix slowly diffuses out to the surrounding fluid. The rate of diffusion depends on the concentration difference (the gradient), the size of the molecule, the temperature, and the resistance of whatever medium or membrane it must cross.

Diffusion is steady but slow over long distances, because it relies on chance collisions rather than any pushing force. That is why it dominates over short distances (across a thin membrane or a stagnant liquid layer) but bulk transport in the body over longer distances is usually handled by flowing blood and fluids rather than by diffusion alone.