One of a group of nitrogenous organic compounds, mostly derived from plants, and having diverse pharmacological properties. They are biosynthesized from amino acids and classified according to some structural feature. A simple classification is into:

the pyridine group (e.g. coniine, nicotine)

the tropine group (e.g. atropine, cocaine)

the quinoline group (e.g. quinine, strychnine, brucine)

the isoquinoline group (e.g. morphine, codeine)

the phenylethylamine group (e.g. methamphetamine, mescaline, ephedrine)

the indole group (e.g. tryptamine, lysergic acid)

the purine group (e.g. caffeine, theobromine, theophylline)