Caffeine

Caffeine is a methylxanthine alkaloid naturally found in various plant parts that acts as an antagonist at central adenosine receptors at relevant physiological concentrations (IC50 = ~30 μM).[1],2 It alters fatigue, mood, alertness, motor reaction time, vascular hemodynamics, and pain sensation.[2],[3],[4] Caffeine has also been implicated in carcinogenesis, although the concentrations used to affect cell cycling and apoptosis in the laboratory may not be commonly achieved in vivo.[5]

Reference:
[1]. Snyder, S.H., Katims, J.J., Annau, Z., et al. Adenosine receptors and behavioral actions of methylxanthines. Proc. Natl. Acad. Sci. USA 78(5), 3260-3264 (1981).
[2]. Fredholm, B.B., Bättig, K., Holmén, J., et al. Actions of caffeine in the brain with special reference to factors that contribute to its widespread use. Pharmacological Reviews 51(1), 83-133 (1999).
[3]. Carrillo, J.A., and Benitez, J. Clinically significant pharmacokinetic interactions between dietary caffeine and medications. Clinical Pharmacokinetics 39(2), 127-153 (2000).
[4]. Pelligrino, D.A., Xu, H.L., and Vetri, F. Caffeine and the control of cerebral hemodynamics. J.Alzheimers Dis. 20, S51-S62 (2010).
[5]. Bode, A.M., and Dong, Z. The enigmatic effects of caffeine in cell cycle and cancer. Cancer Letters 247(1), 26-39 (2007).