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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With medications there are actually two pathways that the body uses. Zero order and first order elimination.
Zero order is simple, over a given time x, y amount of drug will be removed from your body. An example to think about is a famous person signing autographs. They can sign at a constant rate of 2 per minute. If they had 100 fans that’s 50 minutes. The same applies to certain drugs and your liver in zero order elimination. This is how ethanol, omeprazole, and a few others are eliminated.
First order elimination is how probably 95% of the medications are excreted. In this method the drugs are cleared at a rate depending on the initial concentration of the drug. A good example for this would be taking a water jug and putting holes in it. At first the water rushes out really fast, but as the water level drops, the flow slows.
This is a rough idea of the basic concepts behind the elimination of drugs in our bodies.
There are a few that use both methods for different metabolites (intermediates during elimination) but those are beyond this discussion, and in the end are limited by the slowest step (zero or first order).
Please note that in both of these are the RATE of elimination, and actually breaking down the drugs in your body is done by many enzymes and to understand it many of them are split into the zero and first order, however often most will have their own constant *k* that helps determine the actual value.
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