Cannabis has been a traditional medicinal herb in central Asia [1], with reports of such usage back to 4700 B.P. in China, India, Persia, Egypt, Greece and Rome [1]. It is now also cultivated and used as a drug crop in at least 172 countries and territories worldwide [2]. It is classified as Cannabis Sativa, C. Indica and C. Ruderalis based on genetics, phenotypic properties and chemical structure [3]. All the classes have medicinal cannabinoids compounds, but in different proportions. For instance, Cannabis Sativa has a high level of Cannabidiol (CBD), while C. Indica and Rudelaris have high and low levels of Δ⁹-tetrahydrocannabinol (THC) respectively [3,4,5].

Aizpurua-Olaizola, et al. [6] listed 554 compounds identified in Cannabis plants, including 125 cannabinoids and 198 non-cannabinoid compounds like phenols and flavonoids, terpenes and alkaloids [7,8]. Cannabinoids are the active medicinal constituents against the development of numerous conditions. CBD has therapeutic activity against antipsychotic, anti depressive, anxiolytic, antiepileptic, anti spasticity and anti-inflammatory, stroke and hypoxic-ischemic, spinal cord injury, rheumatoid arthritis and various types of cancer, such as brain, blood, breast, lung, prostate and colon [9,10,11,12,13,14]. However, THC is well-documented having anti-inflammatory effects, including for arthritic and inflammatory conditions [15], Alzheimer’s disease [16], Parkinson’s [17] and diabetes [18]. However, cannabigerol (CBG) and cannabichromene (CBC) have antibacterial and antifungal effects [19], and can act as antidepressants [20]. In contrast, cannabinol (CBN) has a potential effect on insomnia and sleep disorder [21].

Production of cannabinoids in Cannabis is mainly derived from cannabigerolic acid (CBGA), or the mother of the Cannabis via co-enzyme Olivetolate geranyl transferase such as tetrahydrocannabinolic acid synthase (THCAS) cannabidiolic acid synthase (CBDAS) or cannabichromene acid synthase (CBCAS) [22], to produce tetrahydrocannabinolic acid (THCA), cannabidiolic acid (CBDA) and cannabichromenic acid (CBCA), and oxidation of THCA to produce cannabinolic acid (CBNA) in the resin glands or trichomes [23,24]. Then these naturally cannebinoids acids are converted via decarboxylation, which removes the carboxylic acid functional group from the cannabinoids via drying, heating or combustion to produce CBD, CBC, CBG and oxidation THC to Delta-8-tetrahydrocannabinol (Δ⁸-THC) and CBN [25,26,27] (Figure 1).

Medicinal Cannabis has traditionally been used to treat various illnesses using different plant parts [28]. According to Jin, et al. [29], leaves are rich in cannabinoids (1.10–2.10%), terpenoids (0.13–0.28%) and flavonoids (0.34–0.44%). The seed oil is mostly used in Arab and Chinese medicine [30,31]. However, a product derived from seed and its concentration still required further study [32]. In addition, a recent study by Lima, et al. [33] reported no spasmolytic effect, no toxicity effect and reduction in edema formation at all tested doses (12.5, 25, 50 and 100 mg/kg) of aqueous extract of Cannabis Sativa roots (CsAqEx) on the airway smooth muscle in mice. In modern medicine, however, Cannabis female inflorescence is the most used part of the plant, and contains the highest concentration of different cannabinoids and active terpenes [34]. C. Sativa species regulated the cell death in the plant via accumulated cannabinoids in the glands above the leaf [35]. Over 300 years, the functions of these glands have been well known as either attracting pollinators or protecting against pathology. However, Cannabis glands contributed additional functions as death capsules [36,37]. These cannabinoids result from secondary metabolism in Cannabis [38]. Cannabinoids accumulate where death happens in the plant tissue [39]. Cannabinoid resin causes necrotic and apoptotic cells through DNA degradation mediated by caspase-dependent nuclease and catalysed by nuclease released from mitochondrial during action mitochondrial permeability transition (MPT) [39]. Secretion of these cannabinoids or treating the plant with these cannabinoids will stimulate the defense system during the initial and late stages of the leaf senescence [36,37]. Another significant benefit is to fulfil the requirement for the completed growth cycle and eventually develop the seeds in the medicinal Cannabis [36]. In general, senescence happens during all growing cycles, and then ultimately leads to death [40]. This death plays an essential role in diverse physiological actions, including root cap, stomatic embryogenesis, xylogenesis, leaf senescence and defense against microbial pathogens and abiotic stresses [41]. Due to secondary metabolites, changes in the active compounds like THCA and THC led to altered chemical composition [38,42].

However, extending the post-harvest life of medicinal Cannabis reduces the rate of metabolism, minimises exposure to post-harvest diseases and limits handling-related damage [43].

Due to decarboxylation, many processes disrupt the chemical composition, such as oxidise or isomerise. [44,45]. Grafström, et al. [46] showed that oxygen increases the decarboxylation of THCA to THC. When inflorescences (buds) are stored under high temperatures. During storage, mostly over 24 h, CBN is formed by the decarboxylation of THC at temperatures over 50 °C [45,47]. However, CBD is at risk of oxidative degradation, and is stable over time [47], since Cannabis is a plant-based drug that, unlike cocaine or heroin, requires no plant processing to produce the active constituents in the administered drug [48].

Post-harvest reports optimising the drying techniques and factors to recover bioac-tive compounds such as terpenes, cannabinoids and phenolic compounds from the medicinal marijuana are limited in the literature, so there is a need to understand the post-harvest handling of Cannabis that optimise yield and control high-quality medical-grade product. Typical processing of Cannabis involves harvesting at the optimal time, drying the harvested product with minimal loss of active constituents, extracting cannabinoids, then storing the product for further process. This review shows that the knowledge for the optimal benefit from the post-harvest process is standardised to produce therapeutically stable Cannabis products for patients by a deeper understanding of its chemical space, outlining major phytochemicals in Cannabis and reviewing the current drying techniques and optimum methods which have been applied for Cannabis-derived phytochemicals. Highly efficient mechanical drying methods are needed to effectively reduce the costs of drying, and phytochemical isolation is crucial for further applications. To date, there are not many papers that discuss the post-harvesting strategies for medicinal Cannabis and managing the quality of export Cannabis.