system
Given the following post, respond as if you're commenting on Reddit:
human
Why are drugs out of our system after five half lives?

Why not 4 or 6?
gpt
They aren't.

After 5 half-lives, an uncomplicated drug will be down to 1/32 of its original concentration. (Because 1/2 * 1/2 * 1/2 * 1/2 * 1/2 = 1/32.)

1/32 is generally considered to be a negligible amount of a medication, because most medications cease to be effective well before their concentration drops by 32-fold. 

However, there are cases when this rule doesn't work — for example, if you overdose, you might be down by 1/32x after 5 half-lives, but you that might still be a toxic amount of drug left.