It is clear from the previous discussion that caffeine and theophylline can affect practically all parts of the cardiovascular system--directly or indirectly. Whether they will and how significant the effect is likely to be is more uncertain. Even when theophylline, which is generally more potent than caffeine, is infused intravenously, the cardiovascular effects are usually limited. Furthermore, tolerance rapidly develops to the cardiovascular effects of caffeine. Hence, in the general population, caffeine taken in common beverages is unlikely to affect the cardiovascular system very much above the fluctuations brought about by the variable stresses of daily life. However, there may be certain individuals who are uncommonly susceptible. For example, those effects that depend on antagonism of endogenous adenosine (and many of the cardiovascular effects seem to do that directly or indirectly) may be particularly strong in individuals who either have high levels of adenosine or usually many adenosine receptors. These may be the individuals who tolerate caffeine-containing beverages poorly.