Order Of Kinetics
FIRST ORDER KINETICS:
 Absorption, distribution, biotransformation and excretion processes are mostly occurring at rates proportional to plasma drug concentration.
 A constant fraction of drug is absorbed, distributed, biotransformed and excreted per unit time.
 These processes increase in rate with increase in conc. and decrease with falling conc.
 A constant fraction of drug in body is eliminated per unit time.
 Rate of elimination is proportional to amount of drug in body.
 For drugs with first order kinetics time required to achieve steady state levels can be predicted from half life.
 The majority of drugs are eliminated in this way.
Half life 
Elimination 
1 t1/2  50% 
2 t1/2 
75% 
3 t1/2 
87.5% 
4 t1/2  93.75% 
5 t1/2  96.875 
FIRST ORDER METABOLISM:
 A drug may be given in doses that produce blood concentrations less than Km of enyzme for drug.
 v = Vmax [C]
 Km + [C]
 When Km >>> [C],
 then v = Vmax [C] & v σ [C]
 Km
 Rate of elimination is directly proportional to drug concentration.
 A constant fraction of remaining drug is metabolized per unit time.
 Most drugs are given at concentrations smaller than Km of enzymes of their metabolism.
ZERO ORDER KINETICS:
 A constant amount of drug is eliminated per unit time.
 Supply of enzymes are limited in body.
 So, with increasing dose enzymes gets saturated at a particular stage.
 At this point there is,
 No elimination.
 Constant biotransformation.excretion.
 So, rate of processes or reaction is not proportional to conc. or dose.
 Processes also referred as “Rate limited/zero order/saturation kinetics/clinically called “Nonlinear kinetics”.
Eg:
 All enzyme mediated processes.
 Drugs involved: Aspirin, ethanol, phenytoin ,propanolol, tolbutamide etc.
ZERO ORDER METABOLISM:
 A drug may be given in doses that produce blood concentrations greater than Km of enyzme for drug.

v = Vmax [C]

K m + [C]

When [C] >>> Km,

then v = Vmax [C] , and v = Vmax

[C]
A constant amount of remaining drug is metabolized per unit time.
Phenytoin undergoes zero order metabolism at doses given.
Drugs showing zero/pseudozero order kinetics :
 Phenytoin
 Theophylline
 Alcohol
 Tolbutamide
 Warfarin
 Salicylates (Aspirin)
 Propanolol
First Order Kinetics

Zero Order Kinetics (Non linear Kinetics)

Exam Question
 Zero order kinetics is independent of plasma concentration.
 Zero order kinetics is otherwise known as saturation kinetics.
 In zero order kinetics constant amount of drug eliminated per unit time.
 Zero order kinetics occur in Phenytoin,Tolbutamide, Propranolol and Ethanol at high dose.
 In first order kinetics a constant proportion of plasma concentration is eliminated.
 In first order kinetics rate of elimination is directly proportional to drug concentration.
 In first order kinetics clearance is constant.
 Elimination after 4 half lives in first order kinetics is 93%.
 Elimination after 3 half lives in first order kinetics is 87.5%.
 For drugs with first order kinetics the time required to achieve steady state levels can be predicted from half life.
 First order kinetics is elimination of the drug is proportional to the serum concentration.
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