Fick’s Law of Diffusion

Fick’s Law of Diffusion, as it pertains to drug movement, states that a drug will flow from an area of higher concentration to an area of lower concentration, with the rate of flow being higher with larger concentration gradients. Thus, a very lipid-soluble drug on the outside of a cell can partition extensively into the lipid bilayer cell membrane, creating a large concentration gradient between the membrane and the aqueous environment on the inside of the cell. This large concentration gradient results in rapid movement of drug from the membrane to the inside of the cell and thus, overall, from the outside to the inside of the cell. A drug which is much less lipid-soluble will partition to a lesser extent into the membrane, generating a smaller concentration gradient and thus a lower rate of movement from the outside to the inside of the cell.

Click here to watch a short animated demonstration of the impact of lipid solubility on the rate of drug diffusion across a membrane.