Metabolism of Caffeine
Caffeine is metabolized in the liver, and how long it takes an individual to clear caffeine from their body is partly dependent on liver function. A general number for the half-life of the drug is 5 hours. This means that half of a dose is metabolized in 5 hours and then half of that in the next 5 hours. Using this average half-life, less than 5% of the drug would remain after 24 hours.
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How quickly the body senses a lack of caffeine largely determines when withdrawal symptoms will start. How tolerant someone is determines how severe the symptoms will be.
With a week of steady use, the body actually changes its makeup and the receptors that caffeine interacts with change in number and function so that the initial effects of small caffeine doses are no longer felt. The rush and nervous energy seen with the first few uses of the drug goes away after tolerance occurs.
The other side of the equation is that these same body changes have to happen in reverse when caffeine is removed from the diet. A good rule of thumb is that it takes as long to get rid of withdrawal symptoms as it does to heal a bruise. Just as a bruise takes time to heal and repair tissue, resetting tolerance level also requires actual changes in cells.
The most common complaint associated with caffeine withdrawal is headache. Because caffeine has vasoconstriction properties (narrowing blood vessels) in the head, stopping use can mean increased blood pressure in the brain, leading to headaches (and rarely nausea).
Another common complaint is a loss of energy and alertness. Simply putting up with this, knowing that the fatigue is temporary and due to withdrawal, usually solves the problem. The sleep cycle may be affected, increasing this symptom.
Mood problems may emerge, with increased irritability, anxiety and even mild depression. Again, knowing the symptoms will be short lived is usually enough to stay the course.
If any set of symptoms becomes too much to bear, small doses of caffeine (60 – 80mg) can sometimes provide relief without extending the withdrawal period more than a day or two.