Certain large, lipid-insoluble drug molecules that resemble amino acids, peptides, or glucose, require specialized carrier proteins to facilitate their diffusion across cell membranes. This transport can occur through either facilitated diffusion, which does not require energy input, or active transport, which does require energy input.

Facilitated diffusion is a passive process that utilizes human Solute Carrier (SLC) transporters. These transporters bind to the drug, undergo structural changes, and then release the drug on the opposite side of the cell membrane.

In contrast, active transport involves moving the drug against its concentration gradient, necessitating energy. Active transport can be further subdivided based on the source of the driving force. Primary active transport engages ABC transporters, which use energy from ATP hydrolysis to enable the unidirectional efflux or outward movement of drugs. On the other hand, secondary active transport employs SLC transporters to move drugs against their concentration gradient. This process is powered by the electrochemical potential generated from the movement of another solute along its gradient. Finally, secondary active transport can be categorized as either symport or antiport. Symport refers to the simultaneous movement of the drug and the other solute in the same direction, while antiport describes their movement in opposite directions.