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

It depends on the chemical composition of the drug and the body’s ability to metabolize it, so there’s variation from one compound to another and even one person to another.

“Half-life” means exactly that: how long it takes for half of the original dose to be detectable. For example, if you consume 40mg of X, you’ve reached its half-life when there are 20mg still present and available. This is usually explained as averages or medians among a group of people studied; that doesn’t tell you how everyone will respond but it’s a sound frame of reference.

Caffeine has a half-life of about 6hrs., but up to 18-20hrs to reach 1/4-life amounts. And this can vary wildly, as some people are very sensitive to caffeine while others never feel any effect at all.

Author Michael Pollan has researched and written about the science and cultural use of caffeine extensively, and you should be able to find several podcasts where he’s featured.

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