The practice of traditional medicine and the use of medicinal plants in most developing countries as a normative basis for the maintenance of good health have been widely reported (Jordan, Cunningham & Marles 2010). Medicinal plants are the backbone of traditional medicine as the world population continually source plant products as their primary sources of medicine (Nunn 2002). Over 80% of Asian and African countries still rely deeply on medicinal plants for their primary health care needs (Yang et al. 2019). Many medicinal herbs are therapeutic at one dose and toxic at another, and with the growing demand for plant-based medicines, there are serious concerns for their use and safety (Saad et al. 2006; World Health Organisation 2013). Despite the increasing number of reports on the medicinal benefits of most plants, the in vivo toxicological implications of treatment with extracts from some of these plants are yet to be investigated (Ekor 2013; Gandhare, Kavimani & Rajkapoor 2013). Hence, there is need to carry out detailed toxicity studies to validate the safety of these herbal medicines in the short- and long-term use.

Irvingia gabonesis belongs to the family Irvingiaceae, and it is an indigenous fruit tree of West and Central Africa (Dienagha & Miebi 2011; Nangue et al. 2011). It is an economically important tree because of its wide use and is known by common names such as wild mango, African mango and bush mango because the tree bears mango-like fruit. The plant is called by different names in different languages of Nigeria: ‘Pekpeara’ in Nupe, ‘Ogwi’ in Bini, ‘Ogbono/Ugiri’ in Igbo, ‘Uyo’ in Efik, ‘Oro’ (tree) and ‘Apon’ (kernel) in Yoruba and ‘Goron biri’ in Hausa (Orwa et al. 2009). Traditionally, it has been reported to be useful in treating ailments such as hernia, yellow fever, gastrointestinal and liver conditions, sterility and urethral discharge, and it is also considered to be an anti-poison agent (Ayuk et al. 1999; World Agroforestry Centre 2004). Bark shavings are used to relieve pain (Okolo et al. 1995) and stop diarrhoea and dysentery, and powdered chocolate prepared from the kernels is applied to burns (Irvine 1961). In an ethno-medicinal study carried out in Ilorin, Nigeria, it has been found that the root bark is also used as a fertility enhancer among males (Nuhu et al. 2018). Despite the ethno-medicinal uses of I. gabonensis in traditional medicine, there seems to be little or no report on the safe utilisation of the root bark. Therefore, this study was undertaken to investigate the safety margins of ethanol extract of I. gabonensis root bark (EEIGRB) in Wistar rats.

Preliminary phytochemical screening gave a brief idea about the qualitative nature of active phytochemical constituents present in plant extracts, which will help the investigators in future regarding the selection of the particular extract for further investigation or isolating the active constituent(s) (Mishra et al. 2011). Different phytochemical agents can trigger the common mechanisms of toxicity at the level of organism, organ, tissue and cell (Rubino 2015). In this study, phytochemical screening revealed the presence of some secondary metabolites that are known to possess numerous pharmacological properties and may be responsible for various activities of EEIGRB. This result is in agreement with the findings of Ojo et al. (2014) who reported the presence of alkaloids, tannin, phlobatannins, saponin, flavonoids, anthraquinones, phenol and cardiac glycosides on the stem bark of I. gabonensis. The acute toxicity studies are important for determining the safe dose of drugs that can be used clinically or experimentally in animals (Gandhare et al. 2013). The result for acute oral toxicity by using OECD guideline 425 limit tests for EEIGRB in rats showed no signs or symptoms of acute toxicity and mortality. This suggests that the LD₅₀ of EEIGRB was greater than 5000 mg/kg body weight in rats. Hence, EEIGRB is considered to be relatively safe and practically non-toxic (Adesegun, Celestina & Coker 2016). This result is in line with the finding of Efosa, Emmanuel and Usunomena (2016), who reported the acute toxicity of ethanol leaf extract of I. gabonensis to be above 5000 mg/kg body weight.

The body weight changes serve as a sensitive indicator of the general health status of animals. Changes in body weight have been used as an indicator of adverse effects of drugs and chemicals (Nandy & Datta 2012). In this research, there was a progressive increase in the body weight of the rats in all the groups over the 28 days and suggesting that the administration of EEIGRB did not exert any deteriorative effect on their normal metabolic processes relating to growth and development.

Organ weight is also an important index of the physiological and pathological status in animals. The relative organ weight is a more viable and sensitive index of toxicity than absolute organ weight as it relates the overall well-being of the animals to each of their organs (Joth et al. 2009). The 28 days of oral administration of EEIGRB showed non-significant changes in relative organ weight when compared with the control rats, which indicate that the extract did not alter the organs of functionality and could be considered non-toxic because decreases in the weights are sensitive markers of toxicity (Waller & Sampson 2018).

The haematopoietic system is a susceptible target for toxic compounds and an important index of physiological and pathological status in man and animals, especially in the bone marrow where the production of RBCs occurs (Kifayatullah et al. 2015). Haematological indices are usually used as markers of toxicity because of the interaction between a toxin and its potential metabolites on cellular components (Arika et al. 2016). The non-significant changes in the haematological indices of the EEIGRB-administered groups relative to the control group in 28 days of administration suggest that it may be nontoxic to the haematopoietic system. However, there was a slight increase in the level of WBCs at 400 mg/kg, which suggest that the extract may contain a biologically active compound(s) that may have activated the immune system (WHO 2004). According to Webb et al. (2003) WBCs help to defend the body against infectious disease and foreign materials as part of the immune system.

Liver function tests involve evaluating serum ALT, AST, ALP, bilirubin and albumin levels. The most commonly used indicators of liver injury are the ALT and AST in the blood stream or plasma, which usually suggests chronic hepatitis or biliary obstructions (Ramaiah 2011). In this study, 28 days of administration of graded doses of EEIGRB did not produce significant effects on the serum levels of liver enzymes, bilirubin and proteins in the experimental animals when compared with control. However, there was a significant increase in the activity of ALP at 400 mg/kg when compared with control. This increase suggests a hepatocellular injury. According to Hassan et al. (2015), increased ALP is a marker for injury to the liver, bone, leucocytes, kidney and intestine.

Creatinine and urea are considered as important prognostic markers of renal dysfunction and kidney failure for any toxic compound (Sood et al. 2015). Amongst the many important functions of the kidney is the maintenance of electrolyte balance. The balance of the electrolytes in human bodies is vital for the normal function of cells and organs (Alhassan et al. 2017). Electrolytes such as sodium and potassium are considered as the key extracellular and intracellular cations, respectively, in the body system (Hassanzadeh-Taheri et al. 2018). In this study, there was a significant (p < 0.05) decrease in the serum creatinine and sodium level after a dose of 400 mg/kg body weight of extract compared with the control group. The altered renal indices may result in impairment of renal tubular reabsorption and glomerular filtration, or excessive secretion of aldosterone hormones, leading to inability to absorb various ions and severe tissue oedema (Yang et al. 2019). This may be because of the phytochemical content in the EEIGRB such as alkaloids, tannins and saponins, which have been reported to be toxic to the liver and kidney (Yakubu & Musa 2012). This suggests that the extract could cause adverse effects on the functions of the kidney if given at high doses and for a long period, and renal function may have to be monitored on long-term administration of the extract. A significant decrease in serum creatinine level usually precedes a renal impairment and muscle wasting (Salawu et al. 2009).

Histopathological examination is one of the gold standards for evaluating treatment-related pathological changes in tissues and organs (OECD 2008). In this study, two vital organs (liver and kidney) were used to assess EEIGRB for toxicity. The liver and kidneys are crucial organs that perform a significant role in detoxification (Hariza et al. 2010). Histopathological examination of the liver revealed a slight hepatic necrosis at the medial dose (200 mg/kg) and a moderate foci necrosis at a high dose (400 mg/kg), which corroborates the finding from the hepatic function indices (Table 2). The histopathological examination of the kidney revealed a slight tubular necrosis at the medial dose (200 mg/kg) and lymphocyte hyperplasia and a slight glomerular distortion at a high dose (400 mg/kg), which corroborates the finding from renal function indices (Table 3) that showed a significant decrease (p < 0.05) in the activities of serum creatinine and sodium.

The EEIGRB was found to be relatively safe after acute administration and showed mild toxicity after 28 days of repeated oral administration. However, EEIGRB was found to have hepatic toxicity and nephrotoxicity with associated histomorphological alterations in the liver and kidneys at an oral dose of 400 mg/kg for 28 days; therefore, prolonged oral administration of EEIGRB should be avoided because of the toxicity risk.