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.

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  • Guy

    After a minor op I threw a caffeine question at the surgeon. He said it dilates the capillaries in the brain and contracts them everywhere else in the body. This would have implications for volume of blood delivered? Can anyone confirm this? Might this cause differences in sensation?

  • Harlan K.

    Guy, one research study did confirm that caffeine is both a vasodilator and vasoconstrictor. However, research so far indicates that caffeine acts as a vasodilator to the brain. This is why caffeine can be effective as a treatment for a migraine headache, as migraines are now thought to be caused at least in part by an increase of blood flow to the brain. Supplements such as L-Arginine and MSM increase blood flow to the brain and cause some people to get headaches. Caffeine’s affects on the blood vessels might vary from one person to the next. I suffer withdrawal headaches when I drink only 6 oz of coffee per day and attempt to decrease to say 4 oz. Most people are not this sensitive.

  • Harlan K.

    haha, I’d love to see some research studies done on caffeine sensitivity over 20 or 30 years, or maybe even the lifetime of people. I don’t recall being very sensitive when I was in my teens and early twenties. Today I have a lot of problems with withdrawal headaches and I can be addicted to just 4 oz of coffee per day. I’ll get a withdrawal migraine headache if I skip my 4 oz of coffee. I actually suspect that if I knew what supplement to take or food to eat I could probably decrease my sensitivity. I’ve searched online a lot to try to find more information about this but I’ve found almost nothing.

  • Debbra

    Thank you for your helpful explanation.

  • Lori Lee

    My physiatrist says drinking coffee w/meds … is like taking an ‘upper’ and says in system for 5 DAYS! What is the scoop!?

  • Ted

    Hi Lori, for most people the half life of caffeine is 4-6 hours. However, caffeine’s metabolites could remain in the system for up to 5 days after a moderate dose.

  • Curious

    So what about coffee enema’s? Are they worth doing? And what are the benefits?

  • Ted

    Here’s our page on the subject.

  • Christina Leigh Richmond

    23 years old, weigh about 180 lbs. Until 3 days ago I drank anywhere from 4 to 6 12 oz Mountain Dews a day, plus at least 1, sometimes 2, cups of coffee every day for at least 5 years. I’ve cut out everything except for water and the occasional glass of milk but I’m still feeling the headaches from what I’m assuming is caffeine withdrawal. I know I’ve got to be in the home stretch with this, but I was just curious as to how long this typically lasts? I know I’m doing my body good by cutting out all the sugar, caffeine and calories that come with my former bad habits, and I’m not taking any kind of medication for the headaches because I know they all have caffeine in them. TIA!

  • Ted

    Hi Christina, Withdrawal can last from 2 weeks to 2 months or more depending on your level of addiction. Hang in there and take comfort in the fact that you’re doing something healthy for your body and others know exactly what you are going through. I also recommend reading our article here:

  • CuriousOne

    Hi – I typically use the bathroom within 30 minutes of my morning coffee. I had the impression that I’m emptying out what may be left in my stomach from the previous night/day. I recently have been subscribed adderall for ADHD, but afraid to take it until after my stomach settles from the bathroom trip as I don’t want it to be flushed out right away. Is this possible, or am I way off base here? I then forget to take my prescription until nearly lunch time when I realize I’m falling behind, and not focused.

  • Ted

    Take your medication. Your digestive tract doesn’t work the way you described above. Also see this article.

  • Jeremy

    I have a question maybe someone here can help, my girlfriend has a few food sensitivities and also has hashimotos, she has energy problems due to this things and she is seeing a specialist to help, she has tried a very all natural “energy drink” and has also drank coffee and other caffeine containing drinks, but never feels the effects of caffeine, she has tested it drinking a lot of caffeine at once, still with no energy effects. Any ideas on why?

  • Ted

    Hey Jeremy, Please see this article:

  • Philip Andrew Reitz

    Is 1 cup of coffee OK a day

  • Philip Andrew Reitz

    Is it OK to eat oatmeal 3 times a day 5 days in a row before a doctor visit

  • Philip Andrew Reitz

    Oatmeal good for u

  • Ted
  • Sage

    Jeremy I suffered from hashimoto’s. Sub-clinical.
    Also Plantar Fasciitis and psoriasis. I cut out caffeine, nicotine, alcohol and gluten(totally). This resulted in a 100% abatement of the plantar fasciitis and 90% abatement of the other two.

    There’s a family history of coeliac going back to the sixties.

  • Jacen Aradaia

    Add a little salt to the oatmeal, eat around 200g steamed kale and 200g free range eggs each day too, and then you will have a healthy, well balanced diet which you could eat indefinitely.

Last Modified: November 9, 2017


  • Biology Online
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