Alkaloids are natural, organic substances that are predominantly found in plants and normally contain at least one nitrogen atom in their chemical structure. Their basic (alkaline) nature has led to the term alkaloids. Since the identification of the first alkaloid, morphine, from the opium poppy (Papaver somniferum) in 1806, more than ten thousand alkaloids have been isolated from plants. Alkaloids are the active components of numerous medicinal plants or plant-derived drugs and poisons, and their structural diversity and different physiological activities are unmatched by any other group of natural products.

Structures of some alkaloids, with the names of the plants that produce them.

Although alkaloids have been detected in some animals (e.g., in the toxic secretions of fire ants, ladybugs, and toads), their major occurrence is in the flowering plants. Alkaloids are relatively stable compounds that accumulate as end products of different biosynthetic pathways, mostly starting from common...