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Dr. C. Rollins
Dr. C. Rollins, Pharm. D.
Category: Pharmacy
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I have a couple of questions related to the Pharmacokinetics

Customer Question

I have a couple of questions related to the Pharmacokinetics of Caffeine. Regarding to how to calculate absorption rate and caffeine in bloodstream.
Submitted: 11 months ago.
Category: Pharmacy
Customer: replied 11 months ago.
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Expert:  Dr. C. Rollins replied 11 months ago.

Caffeine from coffee or other beverages is absorbed by the small intestine within 45 minutes of ingestion and distributed throughout all bodily tissues. Peak blood concentration is reached within 1–2 hours.It is eliminated by first order kinetics.
Vd = volume of distribution
ke = elimination rate constant
ka = absorption rate constant
F = fraction absorbed (bioavailability)
C = plasma concentration

Plasma concentration (single dose)

Time of maximum concentration (single


Customer: replied 11 months ago.
Hi, can you show me an example with numbers... Let's say you had 100mg of caffeine, how much caffeine is in your blood after 3.56 hours.Couple other questions:How do I calculate the "elimination rate constant" and "absorption rate constant"? What does the "Volume distribution" mean?
Expert:  Dr. C. Rollins replied 11 months ago.

The issue with caffeine is that the numbers are different depending on the source of the caffeine. The age, race and weight of the person consuming it. Other medications have a large effect on absorption as well. Following oral consumption, caffeine is absorbed into the blood and body tissues and absorption is virtually complete about 45 minutes after ingestion. The peak plasma caffeine concentration is reached 15-120 minutes after ingestion. Caffeine has a half-life of approximately four hours.

Customer: replied 11 months ago.
What is the Volume of distribution? I'm trying to calculate the plasma concentration for this scenario. You had 100mg of caffeine and the half-life is 4 hours. That means C should equal 50mg at T=4hrs. I'm not getting that when I use the formula.
Expert:  Dr. C. Rollins replied 10 months ago. I found the following webpage that will help you with your question. This is a simple answer to your question. From a pharmacist (kinetics view) it is much more complex, but I don't think that is what you are needing to answer your question. Hopefully this web page explains enough for you to solve your problem. I think it will. Once satisfied please rate my answer. Thanks.