When a drug follows nonlinear pharmacokinetics, its bioavailability, the amount of the drug that reaches the systemic circulation, can change with different doses. This is due to the presence of a saturable pathway. The pathway becomes saturated as the drug concentration increases, decreasing the absorption rate. Consequently, the drug's bioavailability may be lower than expected at higher doses.

To quantify the extent of bioavailability, pharmacologists often use a parameter called . represents the area under the plasma concentration-time curve from time zero to infinity. However, it is important to note that can be influenced by factors such as saturation-limited absorption in the gastrointestinal tract or concentration-dependent AUC affected by the enzymes involved in drug elimination. These factors can further complicate the bioavailability estimation for drugs exhibiting nonlinear pharmacokinetics.

Another factor that can contribute to nonlinear pharmacokinetics is drug-protein binding. When a drug binds to plasma proteins, such as albumin, it forms a complex that affects its distribution and elimination. Protein-bound drugs tend to have a longer elimination half-life and slower elimination rates compared to non-protein-bound drugs. This is because only the drug's unbound (free) fraction is available for elimination, while the protein-bound portion remains in circulation for longer.

In the case of protein-bound drugs, it is important to note that the free drug concentration, representing the unbound fraction of the drug, is always less than the total drug concentration. This is because most of the drug is bound to plasma proteins, limiting its availability for therapeutic action and elimination.

One example of a drug that demonstrates nonlinear pharmacokinetics is valproic acid. Valproic acid exhibits nonlinear protein binding, meaning its binding to plasma proteins is not linearly related to its concentration. This nonlinearity contributes to the complex pharmacokinetic behavior of valproic acid, including its dose-dependent bioavailability and variable elimination rates.

Nonlinear pharmacokinetics, influenced by factors such as saturable pathways and drug-protein binding, add complexity to the understanding and predicting drug behavior in the body. By studying these phenomena, scientists can optimize dosing regimens and enhance therapeutic outcomes for patients taking medications with nonlinear pharmacokinetics.