In first-order kinetics, if 20 percent of the drug is removed every hour, how does the amount removed per hour relate to concentration?

• A. The absolute amount removed per hour decreases with concentration
• B. The absolute amount removed per hour is constant
• C. The absolute amount removed per hour increases with concentration
• D. The absolute amount removed per hour is zero

Answer: (C)

In first-order kinetics, the elimination rate is proportional to how much drug is present, so a fixed fraction removed per hour translates to a fixed fraction of the current amount being eliminated each hour. If 20% is removed in an hour, the amount removed in that hour is 0.20 × current concentration (or amount). Since the current amount scales with concentration, a higher concentration means a larger absolute amount removed per hour, and a lower concentration means a smaller absolute amount removed per hour. This is why the absolute amount removed per hour increases with concentration. The alternative of a constant amount per hour reflects zero-order kinetics, which is not the case here, and a zero amount per hour would only occur if there were no drug present.