How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Dr. C. Rollins Your Own Question
Dr. C. Rollins
Dr. C. Rollins, Pharm. D.
Category: Pharmacy
Satisfied Customers: 647
Experience:  Knowledge in hormone replacement therapy, pediatrics, compounding, long term care, retail, hospice, etc.
Type Your Pharmacy Question Here...
Dr. C. Rollins is online now
A new question is answered every 9 seconds

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: 1 month ago.
Category: Pharmacy
Customer: replied 1 month ago.
Posted by JustAnswer at customer's request) Hello. I would like to request the following Expert Service(s) from you: Live Video Call. Let me know if you need more information, or send me the service offer(s) so we can proceed.
Expert:  Dr. C. Rollins replied 1 month 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 1 month 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 1 month 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 1 month 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 29 days 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.

Related Pharmacy Questions