What does 4–5 half-lives indicate in pharmacokinetics?

• A. Time to reach approximately 50% of steady state.
• B. Time to reach approximately 94–97% of steady state.
• C. Time to eliminate 90% of drug.
• D. Time to reach approximately 63% of steady state.

Answer: (B)

The concept being tested is how long it takes for drug levels to reach steady state with repeated dosing, and how that buildup is governed by the elimination half-life. With each half-life, the remaining distance to steady state is halved, so after n half-lives you’re about 1 − (1/2)^n of the way there. Numerically, four half-lives bring you to about 93.75% of steady state, and five half-lives to about 96.9%. That’s why four to five half-lives indicate a near-complete attainment of steady-state concentrations, roughly 94–97% of steady state.

For context, reaching 50% of steady state happens after one half-life, and reaching about 63% corresponds to about 0.69 half-lives, not four or five. Eliminating 90% of the drug from the body would occur after roughly 3.3 half-lives, which is another rule of thumb but not the one about approaching steady state with dosing.