International Board of Certified Lactation Consultant (IBCLC) Practice Exam

If a drug has a 4-hour half-life, how long will it take to clear a person's system?

1. 12 hours

2. 16 hours

3. 20 hours

4. 24 hours

The correct answer is: 20 hours

To determine how long it takes for a drug to clear from a person's system, it's important to understand the concept of drug clearance in relation to its half-life. The half-life of a drug is the time it takes for the concentration of the drug in the bloodstream to reduce by half. Most pharmacological guidelines indicate that it generally takes about 4 to 5 half-lives for a drug to be eliminated from the system. This is based on the understanding that with each passing half-life, the drug concentration decreases significantly. After the first half-life, 50% of the drug remains; after the second half-life, 25% remains; after the third half-life, 12.5% remains, and so forth. Given a drug with a 4-hour half-life, if we multiply this by 5, we find that: - After 1 half-life (4 hours): 50% remains - After 2 half-lives (8 hours): 25% remains - After 3 half-lives (12 hours): 12.5% remains - After 4 half-lives (16 hours): 6.25% remains - After 5 half-lives (20 hours): about 3