Caffeine Metabolism


Have you ever thought about what is actually going on in your body when you throw back that energy drink or sip that coffee?

How is caffeine broken down and how does it affect your metabolism? How long does caffeine stay active in your system and why do people crave the stuff?
We’ll try to answer those questions and more in the simplest terms possible so that you can be an educated consumer when it comes to caffeine metabolism.

Caffeine From the First Sip

Caffeine easily passes through body membranes so from the first sip the caffeine is entering your bloodstream through the lining of your mouth, throat, and stomach.

It only takes 45 minutes for 99% of the caffeine to be absorbed through these membranes.

In humans, the half-life for caffeine is anywhere from 4 to 6 hours on average, which explains why the average energy drink or coffee’s effect lasts about 4 to 6 hours.

Things like age, medical conditions, and drug interaction can have an effect on the half-life.

Note: Humans also can have 3 levels of sensitivity to caffeine. This also determines how well you metabolize caffeine and to what degree its stimulating properties affect you. Read about those here.

Caffeine in the Blood Stream

While most research on caffeine has been conducted using animals, the data has been converted to show the most likely effect on the human body. As soon as the caffeine enters the body it is already being metabolized by the liver and broken down into theophylline, theobromine, and paraxanthine.

From there these chemicals travel throughout the body where they affect various body functions.

It’s in Your Genes

The speed at which caffeine is metabolized depends on specific genes. Research continues to discover gene variations that appear to be responsible for how long caffeine stays in the bloodstream.

People with a specific variation of the gene PDSS2 process caffeine more slowly than others. They, therefore, need less coffee for the same stimulant effects.

Caffeine in the Brain

The most studied of these is the way caffeine is similar to the molecule adenosine in the brain. The caffeine molecules bind to the adenosine receptors in brain cells and block adenosine from binding.


Adenosine plays a role in the sleep-wake cycle. When adenosine binds to enough receptors, it signals the brain that it is time for rest or sleep. Caffeine doesn’t replace the person’s need for sleep, but just covers up drowsiness symptoms that adenosine can no longer produce.

This also interacts with the dopamine system in the brain, which is the feel-good neurotransmitter. When adenosine is blocked by caffeine, the dopamine system works more efficiently.

Furthermore, elevated levels of adenosine in the blood cause the adrenal glands to release adrenaline. This stimulating hormone further adds to the feelings of alertness and energy.

Here’s a great TED-Ed video that explains what happens in the brain.

Caffeine’s Effects on the Body

Theophylline relaxes smooth muscles, which has been beneficial to those with asthma and is the reason why after drinking caffeine a person often feels the need to use the bathroom as it is affecting the smooth muscles of the colon.  Theobromine increases the amount of oxygen and nutrients that can be used by the brain and muscles.

Caffeine causes an initial contracting of artery walls (vasoconstrictor) but then relaxes this smooth muscle which has an overall vasodilator effect on the blood vessels. In other words, it opens blood vessels to allow for greater blood flow. src.

There have been many other researched effects of caffeine covered by Caffeine Informer, but the above represents the most researched and common.

Look here to find out how much caffeine would be deadly in humans.

Caffeine’s Exit from the Body

The caffeine metabolites are then filtered by the kidneys and they exit the body with the urine.

Caffeine has been shown to have a diuretic effect on the body, which basically causes the body to release more water in the urine.

However, new research suggests that this is only in people who have not built up a tolerance to the caffeine molecule.

When caffeine has exited the body or has been used by the various cells of the body the person can experience a “crash” that is caused by elevated levels of adenosine flooding the brain and dopamine now being repressed. This causes feelings of tiredness and fatigue. Prolonged use of caffeine also causes withdrawal symptoms.

Get Help Quitting Caffeine

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  • Vimugdh

    Ted, I’m 19 years old, don’t drink either tea or coffee as yet-not even one cup in an year! My college is really far and although I leave early in the morning, I return from college in the evening and go off to sleep for around 5 hours-which is all the sleep I get- and wake up at night. I am thinking of taking up drinking coffee so that I may stay awake all night for studying. Would that be fine or should I just have a better diet? Also can I exercise with this kind of routine?

  • Ted

    It sounds like you aren’t getting enough sleep in general. You really should be aiming for at least 7-8 hours per night. Also yes, you should try to eat as healthy as possible and make time for exercise. With that being said, coffee can be used safely for keeping people awake when studying and or driving. I would just advise you not to use it everyday as it can become quite addicting for many. Here are some tips for getting a better night’s sleep.

  • Juergen

    I’m 64 and since I was 13, coffee played a part in my life. 9 tears in the Navy followed by years and years of drinking coffee….2 years ago, I quit a number of things including coffee.
    For whatever reason, 2 weeks ago, I pulled the coffee maker out of the garage. I’d forgotten how much I really enjoyed it. As a result, between 5 and 7 in the morning, I’ve had 4 cups every day. Goya espresso. Good stuff.
    Two days ago, I started having a burning sensation in my thighs, just above my knees. It’s a pretty uncomfortable feeling. Looked the situation up on the web and it appears caffeine could be the culprit.
    Ted, ever heard of this before?

  • Ted

    I’ve heard of all kinds of reactions on our caffeine allergy page. I guess you would have to eliminate the coffee and see if it goes a way and then have it again to see if it comes back. This would help you determine if coffee indeed is the culprit.

  • Dylan

    How many grams of caffeine can the human body take and absorb at any one time?

  • Ted

    There is no limit on this. Caffeine will continue to absorb until it reaches lethal levels which is on average 150mg/kg of body weight.

  • Dylan

    Thanks very much ted, have been wondering that for a long time.

  • Ted

    glad we could help!

  • Dylan

    No your not go Pop a few adderal or some ephedrine and tell me your still a freak when it comes to drugs

  • jose

    Hi ted um im 14 and weight 125lb and drank a energy supplement wich was more than 500mg of caffeine im throwing up and feel really bad

  • Ted

    Drink plenty of water and give it time. If you don’t start improving, get to a doctor. The safe limit for a guy your age is 100mg/day.

  • ryan

    I. Was wondering about taking caffeine pills by insufflation,is this a. Good idea since the liver breaks caffene down. Would I get more. Bioavailability ffrom oral route or insufflation?


  • Ted

    There are several caffeinated products that use this method and the caffeine does make it to the brain faster, so less amounts are needed for effect. See :

    I wouldn’t recommend crushing and snorting caffeine pills. It would be difficult to meter dosage and not sure what damage it would do to your nasal membranes.

  • Adz

    Hi ted … Around 4 hours ago I drank a pre work out drink with 400mg of caffeine I’m 12 and half stone … Roughly how long is it gonna take for it to leave my system and is there anything I can eat or drink to try make me feel any better??

  • Ted

    I’m sure by now you are feeling better. When this happens drink plenty of water and hopefully you learned a lesson? Sorry for the late response but we usually only check in on the comments a couple times a day.

  • Aprilslp

    I have been consuming more caffeine than usual for me lately. It has been something I’ve usually avoided in the past. It’s still not a lot – probably only about 1 cup of coffee and up to 2 sodas per day. I’ve recently developed a twitch in my left eyelid. I’m not certain if the caffeine is to blame, but I’m back to avoiding it again. How long after I’ve stopped drinking anything with caffeine in it would the twitch likely stop if the caffeine had been the cause?

  • Ted

    Actually caffeine is linked to reducing eye twitching or spasms. It’s #9 on our list here
    It’s usually caused by stress. Have you been under more stress lately?

  • Dave

    Ok so im a little confused, is it instant tonget absorbed or does it take 45min or what?

  • Ted

    both, some is instantly absorbed and then it continues to be absorbed for about 45 minutes. At that point 99% has been absorbed.

  • Aaa

    I’ve heard sleep deprivation and stress cause that. I think it happens to me when I don’t get enough sleep. Maybe the caffeine isn’t letting you sleep well?

Last Modified: November 9, 2017


  • Biology Online
  • Tassaneeyakul, W., Birkett, D. J., McManus, M. E., Tassaneeyakul, W., Veronese, M. E., Andersson, T., ... & Miners, J. O. (1994). Caffeine metabolism by human hepatic cytochromes P450: contributions of 1A2, 2E1 and 3A isoforms. Biochemical pharmacology, 47(10), 1767-1776. Link
  • Grant, D. M., Tang, B. K., & Kalow, W. (1983). Variability in caffeine metabolism. Clinical Pharmacology & Therapeutics, 33(5), 591-602. link
  • Echeverri, D., Montes, F. R., Cabrera, M., Galán, A., & Prieto, A. (2010). Caffeine's vascular mechanisms of action. International journal of vascular medicine, 2010. study link