Question:
Which of the following causes of non-linearity is associated with phenytoin?
Which of the following causes of non-linearity is associated with phenytoin?
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Some drugs do not show simple, proportional behaviour as the dose increases — this is called non-linear (or dose-dependent) pharmacokinetics.
Updated On: Jun 24, 2026
- Enzyme induction
- Saturation of metabolic enzymes
- Saturation of plasma protein binding
- Saturation of renal excretion
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The Correct Option is B
Solution and Explanation
Concept:
Some drugs do not show simple, proportional behaviour as the dose increases — this is called non-linear (or dose-dependent) pharmacokinetics. One common reason is that the enzymes that metabolise the drug become "saturated" — they can only work at a maximum speed, so beyond a certain dose they cannot keep up.
Step 1: Phenytoin is the classic example of a drug whose metabolising enzymes get saturated at normal therapeutic doses. Once the enzymes are working at full capacity, a small increase in dose causes a disproportionately large rise in plasma concentration.
Step 2: This is described by Michaelis–Menten (capacity-limited) kinetics. Because of saturation of metabolic enzymes, phenytoin must be dosed very carefully and monitored.
Answer: Option (2) — The non-linearity of phenytoin is caused by Saturation of metabolic enzymes. (2)
Some drugs do not show simple, proportional behaviour as the dose increases — this is called non-linear (or dose-dependent) pharmacokinetics. One common reason is that the enzymes that metabolise the drug become "saturated" — they can only work at a maximum speed, so beyond a certain dose they cannot keep up.
Step 1: Phenytoin is the classic example of a drug whose metabolising enzymes get saturated at normal therapeutic doses. Once the enzymes are working at full capacity, a small increase in dose causes a disproportionately large rise in plasma concentration.
Step 2: This is described by Michaelis–Menten (capacity-limited) kinetics. Because of saturation of metabolic enzymes, phenytoin must be dosed very carefully and monitored.
Answer: Option (2) — The non-linearity of phenytoin is caused by Saturation of metabolic enzymes. (2)
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