Which of the following is true about five half-lives in drug clearance?

Which of the following is true about five half-lives in drug clearance?

• A. 5 half-lives
• B. 3 half-lives
• C. 7 half-lives
• D. 10 half-lives

Answer: (A)

Half-life is the time required for the drug’s concentration to fall to half of its current value. With each half-life, the remaining amount is halved again. After five half-lives, the remaining fraction is (1/2)^5 = 1/32, which is about 3%. That means roughly 97% of the drug has been cleared from the body, so five half-lives is the commonly used threshold for near-complete clearance. For perspective, three half-lives leave about 12.5% remaining, seven half-lives about 0.8%, and ten half-lives about 0.1%.

Half-life is the time required for the drug’s concentration to fall to half of its current value. With each half-life, the remaining amount is halved again. After five half-lives, the remaining fraction is (1/2)^5 = 1/32, which is about 3%. That means roughly 97% of the drug has been cleared from the body, so five half-lives is the commonly used threshold for near-complete clearance. For perspective, three half-lives leave about 12.5% remaining, seven half-lives about 0.8%, and ten half-lives about 0.1%.