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

Reduce your caffeine intake without pain and discomfort.

See our new 10-step plan
  • Palatable

    Now where did i hide that monster?

  • Catty

    Hi, I really want to stop drink coffee but the problem is that everytime I have tried to do it, I suffer from constipation ands tiredness…

    what is a good substitute? Any tips and tricks?

  • ted

    @Catty, it will take some time for your body to re-adjust to having no caffeine after about 3 days to a week you should be back to normal…increase your fiber intake (lots of fruit) and as for tiredness you can take naps or just battle through it. I did a caffeine detox and wrote about it here to reset my caffeine clock so to speak, now I only have coffee a few times a week and it works great.

  • dave

    Catty, take a systematic approach.

    Caffeine depletes the body of electrolytes so as you gradually taper off caffeine, be sure you have ample nutrient intake such as potassium, (moderate) sodium, plus of course magnesium. In other words, eat lots of fruit as Ted already mentioned but also well rounded meals.

    Before you taper back on caffeine much, increase the amount of exercise you get daily. This added strength will offset the tiredness and increase your metabolism too which offsets the effect of stimulant withdrawl.

    Get plenty of sleep and accept that for a day or two your performance doing things will be lower, but don’t stress out over it, everyone has off days.

    Constipation is easily solved from the exercise and drinking additional water. As you decrease caffeine intake your body’s water level will be readjusting anyway so I suggest that you not increase your daily fiber intake any more than necessary as that plus the extra water you should be drinking, together could have the opposite effect of diarrhea.

    Also avoid caffeine free tea, cola, decaf coffee, chocolate. These may not have much caffeine but they still have compounds that your body breaks the caffeine down into so ironically enough it is not very relevant that they’re supposed to be caffeine “free”, they are just a lower dosage of stimulants instead of stimulant free.

    However, caffeine free versions of the above may be one way to help you taper off and decrease stimulant consumption, just be aware that ultimately if you want to avoid caffeine-like stimulants you will have to abstain from these common foods and beverages too.

  • stuart

    Why quit, just cut back to something reasonable like one cup in the morning.
    I used to drink too much coffee and cut back and now I enjoy it.

  • Cat

    Does caffeine also cause anxiety? I am hooked on a large coffee with a pump of hazelnut and soy milk: I drink this every morning. I do notice that my performance has increased, but I’ve also noticed that I’ve felt more ‘tense’ and ‘intense’…. thoughts anyone?

  • ted
  • Nikki

    I drank a 16oz Monster 20 minutes before going to bed…..slept like a baby. Wish I could experience the “uplifting” effects of caffeine. It just seems to make me tired. Does this mean that I metabolized caffeine at an extremely rapid rate? No buzz, just crash!

  • Murray


    That just means your body is used to those high levels of caffeine. I used to be the exact same when i was in college and strung out on rockstars. Now I only drink coffee a couple of times a week and a rock star makes me super hyper.

    Try giving it up for 1-2 weeks and try drinking that same monster and going to bed. It is impossible when your body isn’t used to those levels of caffeine.

  • gen

    Cat, absolutely. Same thing with me, but this doesn’t really happen with instant. It seems to happen more with espresso coffees.
    ‘Tense’, ‘intense’ and paranoid :/

  • Bobby

    Coffee makes me happy

  • Deb

    Got to love the colon effects of caffeine consumption, we all need a little help now and again. For ME the “alertness” effects happen a full 12 hours later making sleep impossible. So I’m very selective about its use. I must say though that first sip of Vanilla Latte sends a wave of pleasure throughout. I’m jealous of you who can indulge regularly.

  • bastard

    shut the fuck up and quit

  • Meeeeee

    What about a cup of tea? I would like to know how long that’ll take to fully take in!

  • Ted

    Hi Meeee. This is the same regardless of the caffeine containing beverage. Since tea has less caffeine the effects are less noticeable.

  • Jared

    How much caffeine does it take to affect you?

  • Ted

    Most people notice effects after about 100mg but this depends on a person’s built up tolerance to caffeine and their sensitivity.

  • Princess Haley Praesent

    I find the time of day also affects weather it makes me tired or energizes me. The brain is pretty complex circuitry and it probably depends on brain neural configuration.

  • Pheonix

    You could easily have ADHD or ADD as well, don’t know anything about you, but stimulants make people with those things drowzy instead of hyped. Does to my boyfriend every time!

  • Science Teacher

    “…caffeine is entering your blood stream through the lining of your mouth, throat, and stomach.”

    Absorption of nutrients doesn’t happen in these places especially in the mouth.

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