What is the minimum number of half-lives needed to achieve steady state plasma levels?

• A. 1-2
• B. 3-5
• C. 7-9
• D. 10-12

Answer: (B)

When a drug is given repeatedly at a constant interval, the amount in the body builds up until input equals elimination, creating a steady-state concentration. Because elimination is governed by first-order kinetics, only a fixed fraction is cleared each half-life, so it takes several half-lives for accumulation to plateau. In practice, about 4 half-lives bring you to roughly 94% of steady state, and about 5 half-lives to around 97–99%. That makes the range of 3–5 half-lives a reasonable and commonly cited minimum to achieve steady-state levels, which is why that answer fits best.