Which substance is commonly used as an example of zero-order elimination?

• A. Morphine
• B. Caffeine
• C. Alcohol
• D. Acetaminophen

Answer: (C)

Zero-order elimination means a fixed amount of the substance is removed per unit of time, regardless of how much is present. This happens when the metabolic pathways become saturated, so the body can’t speed up elimination as concentration rises. Ethanol is the classic example because, at typical drinking levels, the enzymes that metabolize alcohol (like alcohol dehydrogenase) are saturated and remove a constant amount per hour. That leads to a straight-line decrease in blood alcohol concentration over time.

Most other substances, such as morphine, caffeine, and acetaminophen, are eliminated by first-order kinetics in usual doses—the rate of removal is proportional to how much is present, so their concentrations decline exponentially rather than linearly. Only at extreme doses might those pathways saturate, but they’re not commonly described as zero-order drugs.