Alkaloids belong to a chemical class of heterocycled nitrogen-containing bases. Most of them are potent drugs.
Classification is possibel either by their natural occurence in plants as well as their chemical structure. Due to asymmetric substituted carbon-atoms they show optical activity. In most cases only one of the optical isomers has biological activity, whereas the other isomer is biological inactive. Therefor racemates (mixtures of the two isomers) shows half of the activity of the active isomer.
The name "Alkaloid" has been established by C.F.W. Meissner in 1819 because of the basic alkali-like behavior of these plant derivates.