Probably the most straightforward definition of apoisnous plant is that given by Forsyth 1954 "One which give rise to a serious departure from normal health, one small quantity of its seed, root or vegetation is eaten by a creature which is susceptible to its effects". However this definition still leaves much to be desired,as how are "serious departure" normal health and small quantity should be interpreted.

The major poisonous principles found among plants are organic compounds such as alkaloids, diverseness, cardia and cyanogenic glycosides, nitro-containing compounds, oxalates, resins and certain proteins and or amino acids. Some plants also accumulate inorganic elements, largely from the soil, and these too may have serious effects on man.

More than 400 glycosides have been characterized, the best known of which are found in Foxglove. The Coumarin glycosides are vitamin k antagonists and may result in abnormal bleeding. There are many other kinds of glycosides a number of which are considered to be poisonous principles,such as those which prevent the thyroid from accumulating inorganic iodine as found in cabbage, broccoli and all species of Brassica.

The glycosides yielding hydrocyanic acid are called cyanogenic, and the most distributed of these is amygdaline which is commonly found in the seeds of apples and pears and leaves of apricots. The severity of poisoning from cyanide in plants depends on how much free hydrogen cyanide exist in the plant.

Oxalic acid, the only organic acid of plants toxic under natural conditions, it causes kidney failure, can form stones and reduces the proportion of calcium in the blood. The examples of poisoning could be multiplied many times, however those given above were chosen partly to show the variety and range of poisoning and to demonstrate that there is no real way at present the toxic principles can be usefully grouped. The shelves and the Traditional practioner drawers contain Herbal medicines that affect the body very powerfully and are to be prescribed with the greatest care.