A drug has a half-life of 6 hours. If a dose is given every 6 hours, a steady-state drug level would usually be achieved in:

• A. 3 - 5 hours
• B. 10 - 12 hours
• C. 24 - 42 hours
• D. 48 - 50 hours

Answer: (C)

When a drug is given repeatedly, the body reaches a steady-state concentration after several half-lives, because each dose adds amount that is then cleared between doses. With a half-life of 6 hours, one half-life equals 6 hours. It typically takes about 4-5 half-lives for steady-state to establish, which is around 24–30 hours. Because individual variation and practical pharmacokinetics can stretch that a bit, clinicians often consider a window up to roughly 6–7 half-lives, or about 24–42 hours, to encompass near-steady-state attainment.

So, the time frame of about 24–42 hours best fits when steady-state is usually reached with dosing every 6 hours.