How long does it typically take to reach steady state with repetitive dosing?

• A. Approximately 1–2 half-lives.
• B. Approximately 4–5 half-lives.
• C. Approximately 8–10 half-lives.
• D. Steady state is achieved immediately.

Answer: (B)

Steady state with repetitive dosing is reached when the rate of drug input balances the rate of elimination, so the concentrations level off with each dose. With first‑order kinetics, drug accumulates dose after dose until a plateau is approached. In practice, this takes about 4 to 5 half-lives: after four half-lives you’re around 94% of steady state, and after five half-lives about 97–98%. So the typical time to steady state is roughly 4–5 half-lives. Shorter (1–2) half-lives don’t allow enough accumulation to plateau, while much longer (8–10) half-lives exceed what’s usually observed. Steady state isn’t immediate because elimination continues between doses and after dosing starts.