A central nervous system stimulant believed to act through adenosine receptors and monoamine neurotransmitters. It is an adenosine receptor antagonist and adenosine 3,5-cyclic monophosphate (cAMP) phosphodiesterase inhibitor. Thus, levels of cAMP increase in cells following treatment with caffeine. It has been reported to affect cellular calcium levels, releasing calcium from intracellular stores. It overrides the cell cycle effects of various chemicals such as protease inhibitors, preventing apoptosis; and it has been shown to inhibit cellular DNA repair mechanisms.