The Efik‐speaking chiefs of Nigeria used the toxic Calabar bean (P. venenosum Balf.; family Leguminosae). P. venenosum was used in the trial of people accused of witchcraft, sorcery, or murder. Medicine men were used to determine the effective dose of portions administered to the accused. It was believed that the poison would kill only the guilty and spare the innocent. This trial by ordeal was effective because of the toxic nature of the alkaloids contained in P. venenosum. When ingested hesitantly or slowly, as expected of a guilty person, the alkaloids had more time to be absorbed into the bloodstream, leading to death shortly after ingestion. When given to innocent people, they would quickly gulp it, resulting in emesis. In other words, the drug would quickly be vomited before exerting any lethal effects [31, 34, 35].
In Madagascar, a prominent example was the widely used ordeal plant tangena (T. venenifera Poir, synonym of Cerbera manghas (L). from the family Apocynaceae). All parts of T. venenifera are toxic, but the nuts that were used against those accused of various crimes, especially witchcraft – are the most toxic and contain the cardiac glycoside tanghin [32, 33]. T. venenifera use often resulted in a very high number of fatalities, with as many as 6000 people reported dead in one incident [33]. The suspect was made to eat a little rice and swallow three small pieces of fowl’s skin followed by the tangena emulsion. After a few minutes, large quantities of tepid water were given, resulting in long and continued violent vomiting. If the three pieces of skin were expelled, the victim was exonerated, as a rule, and left to be nursed by their friends. T. venenifera has not been pursued for the possible development of any modern drug.
Other plant species used as ordeal poisons across Africa include Strychnos icaja Baillon, whose roots are widely used in ordeal poisons in West and Central Africa [36].
Menabea venenata Baill. (family Asclepiadaceae) was used in a similar manner to T. venenifera, but was dwarfed by its power. It was used exclusively as an ordeal poison by the Sakalave tribe in the arid regions of the west and northwest of Madagascar. M. venenata also contains a powerful cardiac glycoside [33].
Erythrophleum couminga Baill. (family Leguminosae) was also used as an ordeal plant poison. The bark of E. couminga is a highly toxic bark and was used as an accessory poison in ordeal trials throughout Madagascar and the Seychelles [33].
1.4 Toxic Plants in Poisoned Weapons
The use of poisoned weapons has been part and parcel of man’s existence since time immemorial in virtually all parts of the world. Poisoned weapons have been used as a means of obtaining food through hunting game, protecting self from enemies and wild animals, and tribal warfare. Even today, the use of poisoned weapons such as arrows for similar purposes continues, especially in Africa, albeit less frequently [3]. Arrow and ordeal poisons are still considered to be conventional natural sources for future drug discovery [37]. Some of the earliest evidence of the use of arrow poisons is from around 218–2050 BCE in the tombs of ancient Egypt. One arrow had a mainly water‐soluble poison, whose aqueous extract was cardioactive in mice [3].
The bow and arrow is the weapon most used by local tribesmen. Generally, forest dwellers have small bows and mostly wooden‐tipped arrows; savannah people have large bows and arrows with iron tips and mostly complicated barbs. The barbs are wrapped behind with plant material for better adhesion of the poison. Another efficient weapon used in many parts of Africa, particularly Central Africa, is the crossbow [3].
1.4.1 Arrow Poisons
Arrow poisons can be roughly classified into African, South American, and Asian types. Arrow poisons from Africa are predominantly cardiac poisons containing cardenolides, whereas those from South America are almost exclusively muscle‐paralyzing or curarizing poisons and contain alkaloids. Arrow poisons from Asia are mainly cardiac poisons with tetanizing poisons and thus contain cardenolides and alkaloids. With few exceptions, African and most Asian arrow poisons are extremely deadly with no antidote. South American curare poisons, on the contrary, can usually be survived by true antidote or artificial respiration [1, 3]. There are concerns about the rapidly disappearing use of arrow poisons [3, 9].
1.5 Plant Fishing Poisons/Piscicides/Ichthyotoxins
Piscicidal/ichthyotoxic plants are widely distributed throughout the world [38]. The use of plant toxins in fishing was widespread in tropical Africa but is now restricted to remote parts of the continent because it is largely banned [39]. In tropical Africa alone, Neuwinger [39] documented 258 fishing poisons/piscicidal/ichthyotoxic plants from 25 years of field research and concluded that 10–20% of fishing poisons are yet to be discovered. Ten years later, Neuwinger [40] documented an additional 325 fish‐poisoning plants.
Using plant extracts or toxins for fishing involves pounding the plant material and throwing it into shallow pools or sections of small rivers that have been dammed to give relatively still water. The fish are stupefied and, after a short while, float to the surface of the water. They are easily picked up by hand and then eaten without any untoward effects on the health of the consumers. Women have traditionally carried out this activity in Africa [39–41].
The main active compounds are saponins, rotenoids, and diterpene esters. They represent the most important and most common constituents in ichthyotoxic plants and are of great interest to scientists [39, 40]. These biologically active compounds have potential as insecticides and many are widely used in traditional medicine and in preparation of arrow poisons [39]. For example, Cissus quadrangularis is used in Nigeria both as a fish poison and as an arrow poison to kill small birds. Many piscicidal plants from Africa are predominantly from the Leguminosae family, followed by the Euphorbiaceae family [39, 40]. Ichthyotoxic compounds have several other biological activities, such as the anticancer and potent antiviral activities of compounds isolated from Dryopteris fragrans [42] and the antibacterial activity of rotenoids against the ulcer‐causing Helicobacter pylori [43].
The most commonly used piscicidal plants in Africa are Tephrosia vogelii, Mundulea sericea, Euphorbia tirucalli, Gnidia kraussiana, Adenia lobata, Balanites aegyptiaca, Swartzia madagascariensis, Neoratanenia mitis, Tetrapleura tetraptera, and Strychnos aculeata [40]. C. quadrangularis is often used in conjunction with other local plant poisons such as B. aegyptiaca and T. vogelii. The mixture of the three toxic plants is more toxic than any one of the poisons used alone [39].
1.6 Poisonous Plants as Food
Oftentimes, there is no clear line between food plants and toxic and medicinal plants [44]. In many cases, one plant may have particular parts that are poisonous, whereas the other parts are edible or medicinal. A case in point is Abrus precatorius, the seeds of which are highly toxic; however, the leaves are edible and are used in traditional medicine [45]. The seeds of Malus spp. (apple) are also poisonous and contain the toxin amygdalin, which is a cyanogenic glycoside [5]. In some cases, the toxic parts have to be detoxified first through various elaborate and careful processes to render them edible. Although several plant species that are potentially toxic are consumed as food and also used as medicine, this section will focus on a few examples of plant species that are widely used as food and are also known to be toxic.
One of the most prominent examples of such toxic and edible plants is the cycad. Cycads have been widely researched as poisonous food plants. Cycads are indigenous to the tropics and subtropics, where they have been used both as a staple and as emergency food and medicine for various ailments [46, 47]. Natives of particular areas have long been aware of their toxicity, which especially manifests