What does half-life of a drug describe?

• A. The time it takes for the drug to be eliminated from the body
• B. The time it takes for the drug in the body to be reduced by half
• C. The time to reach peak concentration
• D. The time for the drug to be absorbed

Answer: (B)

Half-life describes the time for the drug’s concentration in the body to fall by half. In first-order kinetics, this is a constant, so with each successive half-life the amount halves again. This concept underpins dosing intervals and how long the drug remains in the body, since about 4–5 half-lives are typically needed for near-complete elimination. The half-life is calculated from clearance and volume of distribution (t1/2 = 0.693 × Vd / CL). It’s not the time to reach peak concentration (that’s Tmax), nor the time for absorption, and it doesn’t specify a single fixed time for total elimination.