Pure caffeine is odorless and has a bitter taste. Effects of Caffeine on the Nervous System Caffeine is classified as a central nervous system stimulant . In massive doses, caffeine is lethal. Caffeine enters the bloodstream through the stomach and small intestine and can have its effects as soon as 15 minutes after it is consumed. 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.