Question

Why does a drug that follows zero order kinetics, behave as a first order reaction at low concentrations of the drug?

Answer 1

Zero order reaction is independent of the concenteration of reactant where as the first order reaction depends on the concenteration of one reactant.

The kinetics of a drug is dependent on elimination rate of total plasma concenteration. The zero order kinetics undergo constant elimination despite of plasma concenteration whereas the first order kinetics is directly proportional to the concenteration of the plasma.When the elimination becomes saturated, the first order kinetics will move to zero order kinetics and again if the concenteration falls below at certain level, it turns back into first order reaction.