From time immemorial, plants are known to be sources of medicinal substances used for prevention and treatment of various diseases around the globe. About 80% of the world’s population has accepted plants as an indispensable source of medicinal substances used for maintenance of well-being.
Despite the above-mentioned medicinal benefits of plants, some of their chemical substances are toxic to man when consumed at high, moderate or even at small concentrations, depending on the type of plant. Some of these toxic substances can affect the entire organ system while some only affect specific organs in the system, for example, digitoxin, oleandrin, neriifolin, and thevetin A and B (cardiac glycosides) target the heart. The presiding effect may lean on the wrong identity of the plant, state or condition of the plant, stage of development (growth), plant part used, species, amount consumed, and vulnerability of the victim.
Nigeria, being in the tropical region is known to harbor numerous plants containing high levels of heterogeneous chemical substances (alkaloids, tannins, flavonoids, saponins, anthraquinones, glycosides, proteins, etc.) for protection against predators and adaptation to the harsh tropical environmental conditions. The use of medicinal plants in Nigerian traditional medical practices is currently not properly checked resulting to increased risk of toxicity due to wrong administration especially of those plants that are toxic at low concentrations. Another area of concern is the risk of genotoxicity associated with chronic exposure to most commonly used medicinal plants.
Numerous research works have pointed out chemical constituents or phytochemical content of some poisonous plants. It is necessary to identify plants with toxicity potentials among the plants used for therapeutic purposes. This will help in reducing the rate of plant-associated poisoning or toxicity. It will also help researchers in making decisions on the dose to use in evaluating the therapeutic effect of a toxic plant.
The aim of this review is to provide an index of Nigerian medicinal plants with potentials for toxicity.
Literature in different local and international peer-reviewed journals that featured on the toxicity of medicinal plants found in Nigeria and is used by the populace in any part of the country for medicinal importance was extensively searched. The scientific search engines used in the study were the Google scholar (https://scholar.google.com) and PubMed (www.ncbi.nlm.nih.gov/pubmed). The study database included research articles, books, these and other scientific write-ups, known for their academic rating, covering various aspects of the plant species (Botany, traditional use, pharmacology, or toxicology) dating from 1982 to 2016. The search strategy used, as employed by Félix-Silva et al. contained one or a combination of the following terms “toxic Nigerian medicinal plants” or “toxic medicinal plants in Nigeria” or “Nigerian toxic plants” or “toxic medicinal plants” or “toxicity” “medicinal plants” “review” or “medicinal plants in Nigeria” “toxicity” or “toxic effect” “Nigerian plants” or “poisonous effect” “Nigerian plants” or “toxicity” or “toxic effect” or “poisonous part.” Search results were screened for relevance in the study. Only literature published in English that reported toxicities or poisonous effects (including human, animal, and in vitro studies) of medicinal plants that can be found in Nigeria were included. Information on toxicities of the studied plants and summary of the work was extracted from the consulted literature [Table 1].
Table 1: Results of literature search
The results above show common medicinal plants found in Nigeria having potential toxic effects to either humans or animals at certain concentrations (where specified) according to various scholarly articles reviewed by the present research. 50 plants belonging to 34 families were recorded in this review article. Euphorbiaceae family had the highest number in this research (11.8%) followed by Apocynaceae and Fabaceae (9.8% each), Sapotaceae (5.9%), Cucurbitaceae, Malvaceae, and Rubiaceae (3.9% each), with the rest having 1.9% each.
The parts of the plants found to be toxic include leaves (25.5%), roots (23.5%), stem bark (9.8%), seeds (9.8%), fruits (2.0%), whole plant (2.0%), aerial part (2.0%), and latex (3.9%), with 21.6% accounting for parts not specified by the studies. The identified plant parts were extracted using either organic solvents (ethanol, methanol, hexane, acetone, and petroleum ether) or water (aqueous).
All the plants mentioned in this study are used medicinally in different parts of the country. This could be due to their therapeutic benefits at lower concentrations and toxic effects usually occur at higher concentrations or when an overdose is used. The plants identified in this study mainly caused intoxication through consumption. Organs systems most affected are shown in Table 2. Some of the plants were shown to demonstrate multi-organ toxicity.
Table 2: Organ systems affected by the plants
According to this study, many indigenous plants around us that are used medicinally possess toxicity potentials at certain levels. They have the ability to cause either immediate (acute) toxic effect (Argemone mexicana, Cannabis sativa, Nerium oleander, Jatropha gossypifolia, Blighia sapida, Thevetia peruviana, Luffa aegyptiaca, etc.) or chronic toxicity when used for a long period of time (Morinda lucida, Kigelia africana, Garcinia kola, etc.). Therefore, caution should be exercised when using these plants for especially medicinal purposes.