why the time that drugs like caffeine affect you are measured with “half life” regardless of the amount ingested rather than a constant rate

Question

Does the body not clear drugs like caffeine at a constant rate? If you drink less caffeine does the body clear it less quickly? Or am I not understanding it? Caffeine half life is about 5 hours (via Google), so regardless of if I drink 100mg or 900mg of caffeine, half (50mg or 450mg) will be left in my body 5 hours later? That seems like a pretty drastic difference in the rate of clearance.

In: 509

Answer 1

A lot of drugs are cleared from your body by the liver, as blood flows through it. The less drug is in the blood, the less the liver catches. So as your blood level drops, the slower the liver is at removing it. Thus if it takes an hour for the liver to clear half the drug from your body, it’ll take another hour to clear half of what’s left, and so on.

Hypothetically, a few molecules of drug might linger in your body forever, but as a rule of thumb, most of it is gone in 5 half lives.