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

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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.

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Anonymous 0 Comments

Caffeine is rapidly absorbed into your bloodstream. (within 15 to 45 minutes after ingesting it) and it is cleared by a set of enzymes your liver produces. The rate of metabolism depends entirely on the speed at which your liver can produce those enzymes. Some people produce them quickly. others much more slowly. For a slow metabolizer of caffeine it can take fifteen hours or more to break it down to negligible amounts. For a fast metabolizer maybe nine hours. that is why the metabolic rate is measured in half lives, because there is a wide range. The dosage (amount of caffeine ingested) impacts how much of the effects of the drug you feel, but the rate at which it is cleared by your liver is largely driven by your genetics and nothing else. Once you ingest it, you just have to wait for your liver to break it down.

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