Approximately how many half-lives are needed for most of a drug to be eliminated from the body?

• A. Two half-lives
• B. Four half-lives
• C. Six half-lives
• D. Five half-lives

Answer: (D)

Half-life is the time it takes for the drug’s concentration in the body to fall by half. In first‑order kinetics, the amount remaining after each half-life is halved again, so the decline is exponential.

Compute the remaining amount after several half-lives: after four half-lives, you have (1/2)^4 = 1/16, or about 6.25% of the original dose. After five half-lives, it’s (1/2)^5 = 1/32, or about 3.125% remaining. When we say most of the drug is eliminated, we’re referring to getting the remaining amount below about 5%. Five half-lives meet that criterion, leaving only about 3% of the drug, which is typically considered negligible for clinical purposes. So the best answer is five half-lives.