Phenytoin is a drug that follows zero order kinetics at high doses and first order kinetics at low doses. Suppose, 400 mg phenytoin is present in the body and its rate of elimination at this dose is 200 mg/hr, how much will remain in plasma after 2 hours?
## **Core Concept**
Phenytoin is a drug that exhibits **zero-order kinetics** at high doses and **first-order kinetics** at low doses. Zero-order kinetics means that the rate of elimination is constant and does not depend on the plasma concentration of the drug. In contrast, first-order kinetics means that the rate of elimination is directly proportional to the plasma concentration of the drug.
## **Why the Correct Answer is Right**
Given that at a dose of 400 mg, the rate of elimination of phenytoin is 200 mg/hr, this indicates **zero-order kinetics** because the rate of elimination (200 mg/hr) is constant and does not change with the dose. To find out how much phenytoin will remain after 2 hours, we subtract the amount eliminated over 2 hours from the initial amount. The amount eliminated in 2 hours is 200 mg/hr * 2 hr = 400 mg. However, since we started with 400 mg and the elimination rate is 200 mg/hr, after 2 hours, 400 mg would have been eliminated if it were possible to eliminate more than present.
## **Why Each Wrong Option is Incorrect**
- **Option A:** This option suggests a calculation based on first-order kinetics, which does not apply here since the given scenario describes zero-order kinetics.
- **Option B:** This option might seem plausible but does not correctly account for the zero-order kinetics and the specifics of the scenario provided.
- **Option D:** This option suggests an increase or a different kind of kinetic calculation that does not align with the principles of zero-order kinetics.
## **Clinical Pearl / High-Yield Fact**
A key point to remember is that drugs that follow **zero-order kinetics** have a **limited capacity for elimination**. As a result, small increases in dose can lead to disproportionately large increases in plasma concentration and potential toxicity. Phenytoin is a classic example of such a drug.
## **Correct Answer:** . 0 mg