Steady state is typically achieved after how many half-lives?

• A. 2-3
• B. 3-4
• C. 5-7
• D. 8-10

Answer: (C)

When a drug is given repeatedly, it takes time for the amounts from each dose to balance with what’s being eliminated, so the plasma concentration settles into a relatively constant range—that's steady state. Because elimination follows first‑order kinetics, driven by the drug’s half-life, the system approaches that balance gradually with each half-life. The common rule of thumb is that it takes about five to seven half-lives to reach near full steady state. In this range, the concentration is essentially at steady state (about 97% by around five half-lives, and over 99% by around seven). So five to seven half-lives is the typical time frame for achieving steady state.