First Order Elimination Kinetic

A drug molecule is eliminated when it comes in contact with its elimination mechanism.
The elimination mechanism may be an inactivating enzyme or excretion process.
First-order elimination kinetic is seen when the elimination process is not saturated.

At high concentration of drug, more drug molecules come in contact with elimination mechanism

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More drugs molecule are eliminated. i.e. inactivated or excreted

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Drug concentration decreases

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Fewer molecules come in contact with elimination mechanism

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Fewer molecules are eliminated

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If we give next dose of drug

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Concentration of drug increases

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Again more drug molecules come in contact with elimination mechanism

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Elimination increases

Thus, at higher concentration, more molecules are eliminated, and at lower concentration, fewer molecules are eliminated.
All the properties of the first-order elimination kinetic are different manifestations of this same fundamental process.

Rate of drug elimination is directly proportional to plasma concentration of the drug.
Constant fraction of drug is eliminated at constant interval of time.
Clearance remains constant.
Plasma half-life remains constant.
Plot of plasma concentration against time is curvilinear.
Single dose of drug is eliminated almost completely eliminated after 5 half-lives.

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