First Order and Zero Order Kinetics

First order and Zero Order kinetics

First order kinetics occur when a constant proportion of the drug is eliminated per unit time.

Rate of elimination is proportional to the amount of drug in the body. The higher the concentration, the greater the amount of drug eliminated per unit time. For every half life that passes the drug concentration is halved. For example a drug concentration of 100 and a half life of one hour will reduce to 50 in the first hour, 25 in the second hour and 12.5 in the 3rd hour and so on. Most drugs are eliminated this way. Elimination mechanisms are NOT saturable

Zero order: a constant amount of drug is eliminated per unit time.

For example 10mg of a drug maybe eliminated per hour, this rate of elimination is constant and is independent of the total drug concentration in the plasma. Zero order kinetics are rare Elimination mechanisms are saturable. Examples of zero order elimination include ethanol, phenytoin and salicylates (at high doses)