**1.1 History of medicinal plant use**

 The origin of medicinal plants use had been since time immemorial and traced back to Europe, Egypt, etc. many centuries ago [1]. The first records of knowledge documentation were, however, produced by Shen Nung (a Chinese emperor) 2500 BC ago, describing different recipes of drug preparation from more than 300 medicinal plants for the management of numerous human diseases. Records had it that the use of plants (herbs) as medicine started gaining momentum around 500 BC, though prior to this period, their use was not limited to healing but believed to possess spiritual (ritual) power as well until the advent of scientific era particularly around 1960s when much relevance was played on development of synthetic products based on assumption that they are safer and come with

 little side effects [2]. Despite the aforementioned, the last two decades witnessed a drastic revival in the use and acceptance of phytomedicine by a majority of the people from developing nations (70–90%) as a major source of primary health care. This was also buttressed by WHO's submission, encouraging the discovery and development of lead drugs from plant-based formulations and/or medicines which are believed to be effective and safe [2]. In fact, the development of morphine, quinine, reserpine, ephedrine, etc., from *Papaver somniferum*, *Cinchona*  spp., and *Rauwolfia serpentina* as first set of drugs from medicinal plants brought much popularity and attested to their acceptance and potential use across different parts of the globe especially from Europe and Egypt, with records of well over 900 drugs compiled in history by chain of scientists such as Discorides and Galen [3]. Moreover, it suffices to submit that China is the only country with complete catalog of phytomedicine [2].

Mankind relies on plants and/or its extract, an integral part of traditional medicine (TM) which as a matter of fact is the origin for medical medicine. The knowledge of TM particularly in issues relating to the health of both humans and animals has continued to emerge in many nations of the world. Despite the unproven quality, safety, and efficacy, they are becoming the major source of health care for 80% of the entire population in both developed and developing countries (such as USA, China, India) in disease control, prevention, and management [4]. Interestingly, TM or phytotherapy (traditional system of health care) in the last two decades is being adopted by every region based on the specific sociocultural context illustrating the way medicinal plants (MP) or the inherent secondary metabolites are used, as well as their disparity in the approach to health and diseases. This TM varies from one community to another and notable among them are Acupuncture (Chinese), Ayurveda (Indian), Kampo (Japanese), Unani (Arabian), Basotho (among Africans), etc., some or majority of which had been in existence many centuries even before the advent of modern medicine.

Similarly, the reliance on plants by humankind is not only limited to medicine but also to other basic needs such as food, clothing, and shelter, all produced or manufactured from plant matrices (leaves, woods, and fibers) and storage parts (fruits and tubers) [5]. Medicinally, plant harbors chemicals referred to as the secondary metabolites, which are derived biosynthetically from plant primary metabolites (e.g., carbohydrates, amino acids, and lipids) though might not be directly involved in the growth, development, or reproduction of plants [6]. These secondary metabolites can be classified into several groups depending on their chemical classes [7].

#### **2. Plant secondary metabolite and their therapeutic significance**

Secondary metabolites are organic compound produced and found in all plant tissues to drive metabolic activities, as well as providing self-defense against herbivore and any form of environmental toxicity [8]. Plant is a well-known source of medicinal product for both traditional and modern medicines for the treatment and management of human illnesses. The usage of the plant in this regard is attributed to the presence of secondary metabolites [9]. Apart from the fact that they are widely used in medicine, they are also employed industrially in the production and manufacturing of dyes, drugs, polymers, waxes, glues, fibers, antibiotics, herbicides, insecticides, cosmetics, etc. [10]. In general, secondary metabolites found in plants can be categorized into three major groups including terpenes (cardiac glycosides, carotenoids, and sterols), phenolics (flavonoids and nonflavonoids), and nitrogen-based compounds (alkaloids and glucosinolates).

#### *Pharmacognosy: Importance and Drawbacks DOI: http://dx.doi.org/10.5772/intechopen.82396*

Terpenes are the largest and highly diversified class of secondary metabolites derived as a result of polymerization of isoprenoid unit of five carbon compounds [11]. Based on the five carbon compound used as its building block, it can be subdivided into monoterpenes, sesquiterpenes, diterpenes, triterpenes, tetraterpenes, polyterpenes, and steroids whose precursor is triterpenes. The therapeutic significance of terpenoids from different plants has been reported, e.g., terpenes from eucalyptus oil is known for its antidiabetic property [11], ursolic acid from *Rosmarinus officinalis* and β-sesquiphellandrene from *Piper guineense* are known to be psychoprotective [12]. Antibacterial and antifungal potential of terpenoids derived from *Pilgerodendron uviferum*, *Picea abies* and other plant sources have also been reported [13–15]. Furthermore, a steroidal terpenoids called glycyrrhizic acid elicited anti-inflammatory activity [8].

The phenolics are secondary metabolites that are produced in the shikimic acid pathway of plants involving pentose phosphate through phenylpropanoid metabolization of at least one aromatic ring of hydrocarbon attached to one or more hydroxyl groups [10, 16]. Phenolics are generally categorized into two based on their structure, namely, flavonoids and nonflavonoids. Structurally, flavonoids are derived from two aromatic rings linked to a bridge consisting of three carbons (C6▬C3▬C6) and are sub-divided into six main categories, including flavonols, flavones, flavanones, flavan-3-ols, isoflavones, and anthocyanins. However, the nonflavonoids are subdivided into five main categories, including hydroxybenzoates, hydroxycinnamates, lignans, and stilbenes [17]. Compellingly, wide arrays of pharmacological potentials, such as antidiabetic, antioxidant, antiviral, antimicrobial, anticancer, and anti-inflammatory, have been credited to plant-based phenolic compounds. For example, cyanidin 3-sambubioside and 5-caffeoyl quinic acid derived from the fruit of *Viburnum dilatatum* Thumb. had been found to elicit significant antioxidant and radical scavenging activities while also inhibiting the syndrome-linked complications of postprandial hyperglycemia [16]. Furthermore, plant-based phenolic acids such as garcinone E, kaempferol, resveratrol, syringaresinol, and quercetin are known to be potent anticancer agents [18]. The antiinflammatory, antiviral, and antibacterial potential of phenolics in the management of skin disorder have also been reported [17, 19–21].

 Alkaloids are structurally diversified secondary metabolites derived from nitrogen-based amino acid with nitrogen atom in the heterocyclic ring. Based on the nature of their heterocyclic and building block, alkaloids are classified into different subgroups such as indole, tropane, piperidine, purine, imidazole, pyrrolizidine, pyrrolidine, quinolizidine, and isoquinoline alkaloids [22]. Noteworthy, therapeutic effects have been credited to a wide range of alkaloids from plants. Typical examples from alkaloids are *Callistemon citrinus* and *Vernonia adoensis* reported to elicit antibacterial effects on *Staphylococcus aureus* and *Pseudomonas aeruginosa* [23]. Additionally, alkaloids originating from *Aerva lanata* roots were able to mitigate postprandial hyperglycemia in diabetic rats [24]. The *in vitro* antioxidant activity of *Phoebe declinata* leaves extract has also been attributed to its alkaloid. It was found to inhibit 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical while consequently reducing ferric chloride to ferrous [25]. Furthermore, plant-based alkaloidal compounds such as reserpiline, α-yohimbine, methylaplysinopsin, isoquinoline, physostigmine, and pilocarpine are good psychoprotective agents [12].
