Methylxanthines are a class of purine alkaloids derived from xanthine with one or more methyl groups. Common examples include caffeine (1,3,7-trimethylxanthine), theobromine, and theophylline. They act as central nervous system stimulants by antagonizing adenosine receptors. Found in coffee, tea, cocoa, and kola nuts, they influence alertness, diuresis, and bronchodilation. Properties vary by specific methylation pattern; caffeine is the most studied. They are biosynthesized in plants from purine nucleotides.