According to the five half-life rule, approximately how many half-lives are needed to reach steady state?

According to the five half-life rule, approximately how many half-lives are needed to reach steady state?

• A. Five half-lives
• B. One half-life
• C. Ten half-lives
• D. Three half-lives

Answer: (A)

Reaching steady state in repeated dosing depends on how quickly the drug is cleared, described by its half-life. With each half-life, the amount remaining from previous doses is cut in half, so the body gradually accumulates drug until input equals elimination. The rule of thumb is that this balance is effectively reached after about five half-lives. At that point the concentration is near its steady-state value, and further dosing doesn’t substantially change it. If you look earlier, after only one half-life the level is far from steady state; after a few more, it’s still approaching steady state; only after several half-lives is it essentially stable. That’s why five half-lives is the best estimate for reaching steady state.

Reaching steady state in repeated dosing depends on how quickly the drug is cleared, described by its half-life. With each half-life, the amount remaining from previous doses is cut in half, so the body gradually accumulates drug until input equals elimination. The rule of thumb is that this balance is effectively reached after about five half-lives. At that point the concentration is near its steady-state value, and further dosing doesn’t substantially change it. If you look earlier, after only one half-life the level is far from steady state; after a few more, it’s still approaching steady state; only after several half-lives is it essentially stable. That’s why five half-lives is the best estimate for reaching steady state.