Passive diffusion is a process by which the molecules are transported according to the concentration gradient from higher to lower concentration. The rate of drug transport across the biological membranes is governed by Fick's law of diffusion.

When the drug is absorbed from the site of administration across a membrane to the systemic circulation, the drug will be diluted immediately in the systemic circulation and will maintain a sink condition that promotes more drug absorption.

Factors other than the concentration gradient such as the lipid solubility of the drug and the surface area available for drug absorption can affect the rate of passive drug absorption.

A carrier molecule is involved in the transport of the drug across the membrane. This carrier can be very specific for a particular drug or a group of drugs. Carrier mediated transport is saturable. This means that the rate of drug transport increases as the drug concentration increases until it reaches a plateau at very high drug concentration.

Carrier-mediated transport can be active transport where the drug is transported against the concentration gradient, i.e. from lower to higher concentration. The active transport process requires energy.

The other kind of carrier-mediated transport is facilitated diffusion where the drug transport is according to the concentration gradient, i.e. from higher concentration to lower concentration. The facilitated diffusion process does not require energy.