Caffeine and adenosine receptor. Computer illustration of a caffeine molecule (top left) interacting with an adenosine receptor (grey) in a cell membrane (white, horizontal). Caffeine is a methylxanthine, which are amongst the most widely used stimulant drugs in the world. The stimulant effects are mediated primarily via blockade of adenosine receptors. Xanthine analogues with improved properties have been developed as potential treatments for diseases such as Parkinson's disease.