than acute, e.g. sesquiterpene lactones, iridoids, pyrrolizidine alkaloids, and tannins. Some alkaloid‐bearing plants that are used as base poisons include Strychnos spp., Boophone disticha, Crinum spp., Triclisia dictyophylla, Nicotiana spp., Physostigma venenosum, Sarcocephalus latifolius, and Erythrophleum spp. [3, 26].
Some medicinal plant species used in African traditional medicine have potential or well‐known toxicities. However, some of the toxic effects of plants are not easy to discern because they are either subacute or chronic. This is compounded by the fact that very few African plants have been tested for toxicity. Tamokou and Kuete [27] evaluated 120 African medicinal plants that had been previously screened for their toxic effects and found that about 40% of them were potentially toxic, with symptoms affecting neurological, hepatic, renal, gastrointestinal, and cardiovascular systems.
1.3 Toxic Plants and Justice
1.3.1 Toxic Plants in Capital Punishment
In 399 BCE, Socrates was sentenced to death for corrupting the youth of Athens and failing to recognize the city’s traditional gods. Socrates is perhaps the most prominent victim of poison from hemlock (Conium maculatum), which was the standard form of capital punishment during his time [28, 29]. C. maculatum contains coniine, a polyketide‐derived alkaloid that is poisonous to humans and animals [30].
1.3.2 Trial by Ordeal
Trial by ordeal is a judicial practice by which the guilt or innocence of the accused is determined by subjecting them to an unpleasant – usually dangerous – experience. In some cases, the accused were considered innocent only if they survived the test, or if their injuries healed [9, 31]. Ordeal by poison is peculiar to Africa [31, 32].
There was a substantial variation in the way ordeal poisons were prepared and dispensed across Africa but the basic procedures were similar. The procedure involved a suspect being given some of the poison to eat or drink, depending on the form in which it was prepared or presented. If the suspect vomited the poison, an indication that the subject’s stomach had rejected it, they would be ruled innocent. However, if their system retained the poison, they would be judged guilty and left to die from the effects of the poison [31, 33].
Most of the ordeal poisons were from the Loganiaceae, Apocynaceae, Leguminosae, and Solanaceae families. Other families with one or fewer representatives of ordeal poisons are Combretaceae, Sapotaceae, Euphorbiaceae, Polygalaceae, and Asclepiadaceae. In most cases, poisons were referred to by a local tribal name, which was mainly a general reference to the ordeal. This rendered the classification of ordeal poisons by tribe practically impossible [33]. The medicine men highly guarded their trade secrets, often making it impossible to identify the plant or formula used [3, 33]. Two of the well‐documented cases are of ordeal poisons with Physostigma venenosum and Tanghinia venenifera.
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
