An alkaloid is a nitrogenous organic molecule that has a pharmacological effect on humans and other animals. Alkaloids are found in plants (e.g., in potatoes and tomatoes), animals (e.g., in shellfish) and fungi.

Even though many alkaloids are poisonous (such as strychnine or coniine), some are used in medicine as analgesics (pain relievers) or anaesthetics. Morphine, cocaine, atropine, caffeine, quinine, theophylline, and theobromine are also members of the alkaloid family.

The following block of text modified from the 1911 encyclopedia, with some obsolete text removed, needs merging with the above

In chemistry, Alkaloid is a term originally applied to any organic base, i.e. a nitrogenous substance which forms salts with acids; now, however, it is usual to restrict the term to bases of vegetable origin and characterized by remarkable toxicological or pharmacological effects. Such bases occur almost exclusively in the dicotyledons, generally in combination with malic, citric, tartaric or similar plant-acids. They may be extracted by exhausting the plant-tissues with a dilute acid, and precipitating the bases with potash, soda, lime, or magnesia. The separation of the mixed bases so obtained is effected by repeated fractional crystallization, or by taking advantage of certain properties of the constituents.

The following classification is simple and convenient; the list of alkaloids makes no pretence at being exhaustive.

  • Pyridine group. Piperine; coniine; trigonelline; arecaidine; guvacine; pilocarpine; cytisine; nicotine; sparteine.

  • Tropine group. Alkaloids characterized by containing the tropine nucleus. Atropine; cocaine; hygrine; ecgonine; pelletierine.

  • Quinoline group. The alkaloids of the quina-barks: quinine, &c.; the strychnos bases: strychnine, brucine; and the veratrum alkaloids: veratrine, cevadine, &c.

  • Isoquinoline group. The opium alkaloids: morphine, codeine, thebaine, papaverine, narcotine, narceine, &c.; and the complicated substances hydrastine and berberine.

  • Phenethylamine group. The phenethylamine alkaloids: methamphetamine, mescaline, ephedrine, &c.

  • Indole group. The indole alkaloids: tryptamine, DMT, NMT, &c

In addition to the above series there are a considerable number of compounds derived from purin which are by some writers classed with the alkaloids. There are also reasons for including such compounds as muscarine, choline, neurine and betaine in this group.

The greater number of these substances are of considerable medicinal value; this aspect is treated generally in the article Pharmacology. Reference should also be made to the articles on the individual alkaloids for further details as to their medicinal and chemical properties.