Various cannabis strains have been bred, often selectively to produce high or low levels of tetrahydrocannabinol (THC), a cannabinoid and the plant's principal psychoactive constituent.

Tiny samples of Cannabis also can be identified with precision by microscopic examination of leaf cells and similar features, requiring special equipment and expertise.

[28][29][30] As a result of intensive selection in cultivation, Cannabis exhibits many sexual phenotypes that can be described in terms of the ratio of female to male flowers occurring in the individual, or typical in the cultivar.

Cannabinoids, terpenes, terpenoids, and other compounds are secreted by glandular trichomes that occur most abundantly on the floral calyxes and bracts of female plants.

[60] It remains controversial whether sufficient morphological and genetic divergence occurs within the genus as a result of geographical or ecological isolation to justify recognition of more than one species.

[64] He considered the genus to be monotypic, having just a single species that he named Cannabis sativa L.[a 1] Linnaeus was familiar with European hemp, which was widely cultivated at the time.

[66] Lamarck based his description of the newly named species on morphological aspects (trichomes, leaf shape) and geographic localization of plant specimens collected in India.

Also, woodier stems, alternate ramifications of the branches, narrow leaflets, and a villous calyx in the female flowers were characteristics noted by the botanist.

In the early 20th century, the single-species concept (monotypic classification) was still widely accepted, except in the Soviet Union, where Cannabis continued to be the subject of active taxonomic study.

[52] In 1929, renowned plant explorer Nikolai Vavilov assigned wild or feral populations of Cannabis in Afghanistan to C. indica Lam.

[65] In 1940, Russian botanists Serebriakova and Sizov proposed a complex poly-species classification in which they also recognized C. sativa and C. indica as separate species.

[74] In 1976, Canadian botanist Ernest Small[75] and American taxonomist Arthur Cronquist published a taxonomic revision that recognizes a single species of Cannabis with two subspecies (hemp or drug; based on THC and CBD levels) and two varieties in each (domesticated or wild).

The framework is thus: This classification was based on several factors including interfertility, chromosome uniformity, chemotype, and numerical analysis of phenotypic characters.

[56][67][76] Professors William Emboden, Loran Anderson, and Harvard botanist Richard E. Schultes and coworkers also conducted taxonomic studies of Cannabis in the 1970s, and concluded that stable morphological differences exist that support recognition of at least three species, C. sativa, C. indica, and C. ruderalis.

Several studies of random amplified polymorphic DNA (RAPD) and other types of genetic markers have been conducted on drug and fiber strains of Cannabis, primarily for plant breeding and forensic purposes.

[60] Another paper in the series on chemotaxonomic variation in the terpenoid content of the essential oil of Cannabis revealed that several wide-leaflet drug strains in the collection had relatively high levels of certain sesquiterpene alcohols, including guaiol and isomers of eudesmol, that set them apart from the other putative taxa.

One laboratory at the University of British Columbia found that Jamaican Lamb's Bread, claimed to be 100% sativa, was in fact almost 100% indica (the opposite strain).

Legalization coupled with Canadian government (Health Canada) oversight of production and labelling will likely result in more—and more accurate—testing to determine exact strains and content.

Cannabis aficionados recognize three distinct types based on such factors as morphology, native range, aroma, and subjective psychoactive characteristics.

[83] Cannabis likely split from its closest relative, Humulus (hops), during the mid Oligocene, around 27.8 million years ago according to molecular clock estimates.

[94] His (c. 440 BCE) Histories records, "The Scythians, as I said, take some of this hemp-seed [presumably, flowers], and, creeping under the felt coverings, throw it upon the red-hot stones; immediately it smokes, and gives out such a vapour as no Greek vapour-bath can exceed; the Scyths, delighted, shout for joy.

Its structure and chemical synthesis were achieved by 1940, followed by some of the first preclinical research studies to determine the effects of individual cannabis-derived compounds in vivo.

[109] However, everyday use of cannabis may be correlated with psychological withdrawal symptoms, such as irritability or insomnia,[105] and susceptibility to a panic attack may increase as levels of THC metabolites rise.

[112] Risk of adverse outcomes from cannabis use may be reduced by implementation of evidence-based education and intervention tools communicated to the public with practical regulation measures.

Cannabis is used to reduce nausea and vomiting during chemotherapy, to improve appetite in people with HIV/AIDS, and to treat chronic pain and muscle spasms.

[118] Two extracts of cannabis – dronabinol and nabilone – are approved by the FDA as medications in pill form for treating the side effects of chemotherapy and AIDS.

Cannabis for industrial uses is valuable in tens of thousands of commercial products, especially as fibre[121] ranging from paper, cordage, construction material and textiles in general, to clothing.

[126] The Yanghai Tombs, a vast ancient cemetery (54 000 m2) situated in the Turfan district of the Xinjiang Uyghur Autonomous Region in northwest China, have revealed the 2700-year-old grave of a shaman.

[127] Near the head and foot of the shaman was a large leather basket and wooden bowl filled with 789g of cannabis, superbly preserved by climatic and burial conditions.

Since the 13th century CE, cannabis has been used among Sufis[147][148] – the mystical interpretation of Islam that exerts strong influence over local Muslim practices in Bangladesh, India, Indonesia, Turkey, and Pakistan.