Once in the body, caffeine will stay around for hours - it takes about 6 hours for one half of the caffeine to be eliminated. Caffeine belongs to the xanthine chemical group. Adenosine is a naturally occurring xanthine in the brain that is used as a neurotransmitter at some synapses. One effect of caffeine is to interfere with adenosine at multiple sites in the brain including the reticular formation. Caffeine also acts at other site in the body to increase heart rate, constrict blood vessels, relax air passages to improve breathing and allow some muscles to contract more easily.
Some studies show that caffeine causes physical dependence.
Typical withdrawal symptoms associated with caffeine are headache, fatigue and muscle pain. These symptoms can occur within 24 hours after the last dose of caffeine. One study has stated that the minimum consumption of caffeine for physical dependence is 4 cups of coffee per day. Other studies say that a few more cups of coffee are needed to develop dependence. One night, Khaldi's goats did not return home.