Tuesday, June 11, 2013

Kinetics of drug elimination simplified

Hi everyone!
We will be covering kinetics of elimination today. =)
Let's get started!
What is first order (exponential) kinetics?
The rate of elimination is directly proportional to the drug concentration.

What is zero order (linear) kinetics?
The rate of elimination remains constant irrespective of drug concentration.

[So zero order doesn't care about concentration or anything. It's just doing a constant amount of work (elimination) not caring about how much work load is left (concentration) :P
First order on the other hand is very organized and according to the amount of work load (concentration) does the work (elimination)
Probably that's why comes first in the working competition ;) ]

In zero order kinetics, a constant amount of drug is eliminated per unit time.
In first order kinetics, a constant fraction of drug is eliminated per unit time.

I remember basic maths here!
You can't make a fraction with zero.
(Anything divided by zero is undefined)
So zero order is relating to constant amount, not fraction.

That will hopefully also let you remember the number one is all about fractions!
You can divide by one and eliminate constant fractions in first order kinetics :D

Most drugs are eliminated by first order kinetics.
So you don't really need to worry about memorizing those.

Zero looks like a pea.
Drugs that are eliminated by zero order kinetics, which are:
Phenytoin Ethanol Aspirin

Cl = 0.7 X Vd / t1/2
Cl: Clearance
Vd: Volume of distribution
t1/2: Half life

In first order kinetics, t1/2 remains constant.
Because volume of distribution and clearance do not change with dose.

In zero order kinetics, t1/2 increases with dose.
Clearance decreases as dose is increased.

That's all!

Hug someone around you today =)



  1. Drugs which have saturation kinetics initially follow a linear line, but then their metabolism slows down leading to a plateau of the line, for example due to enzyme depletion. Small doses in the drug then lead to large increases in plasma concentration.

    This response is typical of drugs such as phenytoin (saturates liver metabolism).

  2. I have another mnemonic for drug which follow zero order..

    " zero watt power"
    Power- phenytoin


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