First Order Elimination:For drugs undergoing first order elimination,a constant FRACTION of the remaining amount of drug is eliminated per unit time dC dt =KC C=Co(erket) Where C.initial drug concentration;C,drug concentration at a given time,t; K=the drug's elimination constant;and t=the time interval from time 0 to time t. LUME人R SCALE LOGARITHMC SCALE Elimination half-life(t1/2)is the time C C at any time,t required to reduce the plasma drug C。=Cat time zero eimination rate constant concentration by 50%.It can be calculated from the elimination rate constant,but it is usually determined from the plasma drug concentration curve (Figure).The half-life can also be expressed in terms of the drug's clearance and volume of distribution. indicating that the drug's half-life will change when either of these factors is altered.The formula for relating half-life to clearance and volume of distribution is given: t1/2=0.693/keElimination half-life (t1/2) is the time required to reduce the plasma drug concentration by 50%. It can be calculated from the elimination rate constant, but it is usually determined from the plasma drug concentration curve (Figure). The half-life can also be expressed in terms of the drug's clearance and volume of distribution, indicating that the drug's half-life will change when either of these factors is altered. The formula for relating half-life to clearance and volume of distribution is given: t1/2 = 0.693/ke