Overview
“Half-life” is the period of time it takes for the amount of some substance to reduce by half.
You might be wondering why we use “half-life” to measure the duration of a drug instead of its total lifetime or an hourly rate, which might be more intuitive.
In many chemical reactions, the concentration of substances follows exponential decay closely, which means that it disappears quickly at first, but the rate of reactions slows down over time. Imagine a room filled with 100 balls that are constantly bouncing around. Let’s say each collision between any two balls results in the balls disappearing. At first, we might imagine a lot of balls would disappear because there are a ton of balls bouncing in every direction. It only takes 5 collisions to go from 100 to 90. However, we can hopefully imagine that it would take a lot longer for 10 balls to go to 0 since collisions are way less likely to happen as more space clears for balls to bounce without collisions. We couldn’t reliably issue a metric like “10 balls will be gone every hour” because of this dynamic. What we typically can say instead if “half the balls disappear every hour.”
Replace these balls with drug molecules, and you arrive at the rationale for describing the duration of the effects of drugs through a half-life.
As an example, you might’ve heard of reports about re-narcotization, in which someone suffering from a fentanyl overdose is given naloxone (Narcan) to reverse the drug’s effects. However, naloxone’s half-life is substantially shorter than fentanyl’s, which means that someone can still overdose as a result of the existing fentanyl in their system that persists after the naloxone is processed. It’s still a life-saver, but it’s not a silver bullet.