Adenosine also dilates blood vessels in cerebral and coronary circulatory networks. Caffeine competes with adenosine for binding at its high-affinity receptor sites, preventing adenosine's normal tranquilizing or sedating effects. Thus, caffeine's stimulant or anxiety-inducing actions appear to be secondary effects of adenosine antagonism. Chronic caffeine exposure has also been shown to induce heterologous up-regulation of adenosine receptors in humans. Caffeine's antagonism to vasodilation by adenosine may account for caffeine withdrawal headaches, as well as for its efficacy in treating migraine headaches.
Caffeine is a mild stimulant and people have variable sensitivity to caffeine.
Caffeine does not accumulate in the blood and is normally excreted within several hours. This may lead to misdiagnoses of Panic or Generalized Anxiety Disorder. Caffeine may impair sleep in sensitive individuals and is not advised at night in insomniacs. In addition to other psychiatric or medical conditions, concomitant use of other psychoactive agents, such as nicotine, alcohol, or benzodiazepines, is common in caffeine users. This often confounds an accurate diagnosis.