Senna (from Arabic sanā), the sennas, is a large genus of around 250 species of flowering plants in the family Fabaceae, subfamily Caesalpinioideae. This diverse genus is native throughout the tropics, with a small number of species reaching into temperate regions. Almost all species were at one time or another placed in Cassia, a close relative which until recent decades served as a "wastebin taxon" to hold all Cassiinae. The species were reassigned by Howard Samuel Irwin and Rupert Charles Barneby, but this process is not entirely complete and some corrections may still take place.
Senna species make good ornamental plants and are used for landscape gardening. The wide variety of species and ecological adaptations makes at least a handful of sennas suitable for any climate warmer than cool-temperate.
Cassia gum - a commonly-used thickening agent -, despite its name is actually from Chinese Senna (S. obtusifolia) seeds. In some Southeast Asian cuisines (particularly those of Thailand and Laos), the leaves and flowers of Siamese Senna (S. siamea, called khi-lek in Thai), either fresh or pickled in brine, are used in cooking, particularly in gaeng khi-lek (khi-lek curry).
Another senna, Senna italica ssp. italica (= Cassia obovata), often called "neutral henna", is used as a hair treatment with effects similar to henna but without the red color. The active component is an anthraquinone derivative called chrysophanic acid, which is also found in higher concentrations in rhubarb root. It adds a slight yellow color.
Some species of Senna are notable for being host to caterpillars of certain Lepidoptera species, for example:
Sennas have for millennia played a major role in herbalism and folk medicine. Alexandrian Senna (S. alexandrina) was and still is a significant item of trans-national trade e.g. by the Ababdeh people and grown commercially, traditionally along the middle Nile but more generally in many regions around the northwestern Indian Ocean.
Sennas act as purgatives and are similar to aloe and rhubarb in having as active ingredients anthraquinone derivatives and their glucosides. The latter are called sennosides or senna glycosides. Senna acts on the lower bowel, and is especially useful in alleviating constipation. It increases the peristaltic movements of the colon. The plants are most often prepared as an infusion. Senna glycosides are listed as ATC code A06AB06 on their own and A06AB56 in combined preparations.
As regards other chemicals, the antiinflammatory compound resveratrol was first isolated from S. quinquangulata, and Siamese Senna S. siamea contains barakol used to counteract aconitine poisoning. Chinese Senna (S. obtusifolia) seeds are also used in Kampō (traditional Japanese medicine) where they are called ketsumei-shi (ケツメイシ, 決明子) or by their Chinese name jué míng zǐ (traditional: 決明子, simplified: 决明子).
The long-standing use of (mainly) Alexandrian Senna is reflected by its presence in many herbal remedies and tonics. These include for example Black draught, Catholicon, Daffy's Elixir, Diasenna (literally meaning "composed of senna") and Swedish bitters. On the other hand, it was contained in more dangerous "medications" such as the highly toxic antihelminthic Lumbricide and - because their purgative effects are a readily-observed "proof" that some concoction "works" - many generally useless and often poisonous "patent medicine".
Today, because of the presence of ample anthraquinones, sennas are still used as the primary ingredient in certain commercial stimulant laxatives. Senna is also the primary ingredient found in most "dieter's teas". The combination of acting as a stimulant which reduces a dieter's appetite, and the laxative properties that cause food to move through their system before as many calories can be absorbed is a combination that can lead to rapid and even dangerous weight loss.
The stimulant action of sennosides should be taken into account for those who suffer from any conditions where stimulants are contraindicated, such as past heart disease, high blood pressure, anxiety attacks, etc. A (generally invisible and harmless) side effect of taking Senna medication regularly is Melanosis coli, a brown discoloration of the colon wall.