For a drug with first-order kinetics, the time to reach steady state is approximately how many half-lives, and what does it depend on?

• A. About 4–5 half-lives; depends on t1/2 and dosing pattern.
• B. About 2 half-lives; depends on dosing frequency.
• C. It is immediate for IV infusion.
• D. About 8–9 half-lives; depends on volume of distribution.

Answer: (A)

In first-order kinetics, the approach to steady state is governed by the drug’s half-life. Each half-life reduces the amount in the body by half, so after about four to five half-lives the average concentration stabilizes and steady state is essentially reached. The key point is that the time to reach steady state is set by t1/2; dose amount or dosing interval determines how high the concentrations rise or fall around that steady state, but they don’t change the overall timeframe to get there. The other statements are not correct: steady state is not immediate with IV infusion; two half-lives are not enough to reach steady state; and tying the time to eight or nine half-lives to the volume of distribution distracts from the central rule that half-life controls the approach to steady state.