First order kinetics describes a process where the rate of reaction is directly proportional to the concentration of the reactant. The rate increases linearly as the concentration increases. The concentration decreases over time but the reaction is never complete. Drug metabolism often follows first order kinetics, where a constant fraction of the drug is metabolized per unit of time. This can be expressed by the rate equation dC/dt = -kC, where C is the concentration and k is the rate constant. The half-life of a first order reaction is calculated from the rate constant. Pseudo-first order kinetics can also describe drug elimination from tissue where high concentrations of tissue components interact with drugs.