Under what circumstances does zero-order elimination occur?

• A. At very low concentrations.
• B. Elimination is always saturable.
• C. When elimination pathways become saturated (Vmax reached).
• D. Only for ethanol.

Answer: (C)

Zero-order elimination happens when the body's capacity to eliminate the drug is saturated. In this situation the enzymes or transporters responsible for clearance are working at their maximum rate, so the amount eliminated per unit time becomes essentially constant and does not depend on how much drug is present. In terms of kinetics, when the concentration is high relative to the enzyme’s Km, the rate approaches Vmax, making dC/dt roughly constant. Consequently, the drug concentration in the blood falls linearly over time rather than exponentially.

This often occurs for drugs like ethanol at typical intoxicating levels, where alcohol-degrading enzymes are saturated, but it’s not exclusive to ethanol. At very low concentrations, elimination tends to follow first-order kinetics, where the rate is proportional to concentration. The statement that elimination is always saturable isn’t accurate, since saturation only happens when the metabolic capacity is exceeded. The correct circumstance is specifically when elimination pathways become saturated and operate at their maximum capacity (Vmax reached).