The half life is the time required for the amount of the drug or the plasma concentration of the drug to decrease by 50% . The half life has units of time.

The half life and the first order elimination rate constant are dependent on the total body clearance and the volume of distribution.

The first-order elimination rate constant is the rate constant for the elimination of the drug from the body. The elimination rate constant and the half life are dependent on the total body clearance and the volume of distribution of the drug.

This rate constant represents drug elimination through all routes of drug elimination such as metabolism, renal excretion, and other routes.

The first-order elimination rate constant has units of time^-1.

Cp₁ and Cp₂ are the plasma drug concentrations at t₁ and t₂, where t₂ > t₁ (amount/volume).