Stimulants bring the consumer in a cheerful mood, increase the appetite and digestion, makes incensitive for physical and mental fatigue, and are usually habit-forming. They contain alkaloids, which are produced in certain plant parts. Alkaloids represent a group of closely related chemical compounds, each with a circular structure containing nitrogen. They are found in the entire plant kingdom, but especially in higher plants. Some of these alkaloids are of large economic importance, like caffeine and nicotine; others are of importance in the pharmaceutical industry. See also Medicinal plants.

Important caffeine containing stimulants are:
- tea, Camellia sinensis;
- coffee, Coffea arabica;
- cacao, Theobroma cacao;
- maté, Ilex paraguariensis.

Nicotine occurs mainly in tobacco, Nicotiana tabacum and in nicotine tobacco, Nicotiana rustica, see Nicotiana tabacum.

The opium poppy, Papaver somniferum, provides opium, which contains a mix of intoxicating alkaloids, amongst which codeine and morphine. Heroine is a derivative from opium.

Cocaine (creating insensitivity for fatique), enables people to continue without food or drink. It is prepared from the leaves of the coca shrub, Erythroxylum coca and it is known since time immemorial to the South American Indians.

Hemp, Cannabis sativa (Stimulants 1), contains cannabinol, that provides the consumer with a sense of well being.

Other stimulants, less known in the western world, are kola nuts, Cola nitida, chewed in West Africa; betel nuts, Areca catechu, chewed in East Asia, and khat leaves, Catha edulis, chewed in Arabia and Ethiopia.