First order elimination: a constant FRACTION of drug is metabolized per unit time (i.e. elimination rate is proportional to the drug concentration)

This differs from zero order elimination where a constant amount of drug is metabolized per unit time (i.e. rate stays constant)

In this question:

• In two hours, 2.5 mg of 12.5 mg is metabolized, which is a fraction of 2.5/12.5 = 0.2 or 20%
• Thus, in another two hours, another 20% will be metabolized since this is first-order elimination
• 20% of 10 is 2 and 10-2 = 8

FA2020 p232