Organic nitrogenous bases. Many alkaloids of medical importance occur in the animal and vegetable kingdoms, and some have been synthesized. (Grant & Hackh's Chemical Dictionary, 5th ed)

A group of basic organic compounds, containing nitrogen and often oxygen, occurring in fungi. Many are poisons and others provide sources for beneficial drugs.

Organic compounds produced by plants. These substances constitute the active ingredients of many drugs, e.g. nicotine, morphine, quinine, caffeine.

Nitrogen- containing organic compounds of natural origin that act as bases in solution; examples are nicotine, quinine, morphine, and lysergic acid.