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
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.
This is specifically first order half life elimination. This is due to to elimination mechanisms being first order meaning that the more that’s in you the faster the rate of reaction for the elimination process is. Some drugs follow a 0 order half life trend, or in other words they are eliminated at a constant rate regardless of the amount.
An ELI5 analogy is you’re in a room full of balloons and you are trying to pop them all. At first there’s plenty of balloons that are right next to you that you can quickly pop without moving. However after popping the ones next to you you will have to move to pop more making it take longer. The more balloons you pop the more you have to move around to pop them increasing the time it takes even more.
Because that’s how your body processes the vast majority of substances. Very few substances are processed at a constant rate. A notable common exception is ethanol (alcohol) which is eliminated at a constant rate.
Trying to think up a very ELI5 analogy
Imagine your blood vessels going through your liver are a big slide. The caffeine in your blood is represented by hundreds of little balls rolling down that slide while your liver removing it is dozens of little holes spaced out in the slide.
While there are loads of balls rolling down, a large number fall through the holes as the slide is full of balls so some always go over the holes and fall in. As the number of balls drop, the slide is less full and less fall in. When few balls are left, lots of space in the slide and the holes are small so the balls can pass by with only a few falling in.
[deleted]
The instantaneous clearance rate depends on the instantaneous concentration at any given point in time. The clearance rate decreases as the concentration decreases.
The half life is the time required to clear half of whatever amount is present.
If the starting concentration is 100, in one half-life the concentration will be 50, on another half life it will be 25, etc.
It’s not just drugs. Most concentrations of substances reach homeostasis at a logarithmic-ish rate across semi-permeable membranes.
There are certain constant rates that you can use to approximate your metabolism. For example, it takes roughly one hour to metabolize a single serving of alcohol. This isn’t exactly true, but it’s close enough to effectively pace your consumption.
Imagine I hide 1000 coins in your house for you to find. After the first hour, you found 100. Do you think you will need an hour for the last 100 coins as well? The last few coins you might actually never find (or, give up the task beforehand.)
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.
Imagine your job is to pick out rice from wheat, or something similar. When there is a lot of rice, it is easy to remove a lot of it. But less rice there is in the wheat, slower you would remove it
First you can pick out 10 grains per minute, but you might spend half an hour searching for the last grain
Latest Answers