The genus Cannabis was formerly placed in the nettle family (Urticaceae) or mulberry family (Moraceae), and later, along with the genus Humulus (hops), in a separate family, the hemp family (Cannabaceae sensu stricto).
Recent phylogenetic studies based on cpDNA restriction site analysis and gene sequencing strongly suggest that the Cannabaceae sensu stricto arose from within the former family Celtidaceae, and that the two families should be merged to form a single monophyletic family, the Cannabaceae sensu lato.

Various types of Cannabis have been described, and variously classified as species, subspecies, or varieties:
plants cultivated for fiber and seed production, described as low-intoxicant, non-drug, or fiber types.

plants cultivated for drug production, described as high-intoxicant or drug types.

escaped, hybridised, or wild forms of either of the above types.

Cannabis plants produce a unique family of terpeno-phenolic compounds called cannabinoids, some of which produce the "high" which may be experienced from consuming marijuana.
There are 483 identifiable chemical constituents known to exist in the cannabis plant, and at least 85 different cannabinoids have been isolated from the plant.
The two cannabinoids usually produced in greatest abundance are cannabidiol (CBD) and/or Δ⁹-tetrahydrocannabinol (THC), but only THC is psychoactive.
Since the early 1970s, Cannabis plants have been categorized by their chemical phenotype or "chemotype", based on the overall amount of THC produced, and on the ratio of THC to CBD.
Although overall cannabinoid production is influenced by environmental factors, the THC/CBD ratio is genetically determined and remains fixed throughout the life of a plant.
Non-drug plants produce relatively low levels of THC and high levels of CBD, while drug plants produce high levels of THC and low levels of CBD.
When plants of these two chemotypes cross-pollinate, the plants in the first filial (F₁) generation have an intermediate chemotype and produce intermediate amounts of CBD and THC.
Female plants of this chemotype may produce enough THC to be utilized for drug production.

Whether the drug and non-drug, cultivated and wild types of Cannabis constitute a single, highly variable species, or the genus is polytypic with more than one species, has been a subject of debate for well over two centuries.
This is a contentious issue because there is no universally accepted definition of a species.
One widely applied criterion for species recognition is that species are "groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups."
Populations that are physiologically capable of interbreeding, but morphologically or genetically divergent and isolated by geography or ecology, are sometimes considered to be separate species.
Physiological barriers to reproduction are not known to occur within Cannabis, and plants from widely divergent sources are interfertile.
However, physical barriers to gene exchange (such as the Himalayan mountain range) might have enabled Cannabis gene pools to diverge before the onset of human intervention, resulting in speciation.
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.
The genus Cannabis was formerly placed in the nettle family (Urticaceae) or mulberry family (Moraceae), and later, along with the genus Humulus (hops), in a separate family, the hemp family (Cannabaceae sensu stricto). Recent phylogenetic studies based on cpDNA restriction site analysis and gene sequencing strongly suggest that the Cannabaceae sensu stricto arose from within the former family Celtidaceae, and that the two families should be merged to form a single monophyletic family, the Cannabaceae sensu lato.
Various types of Cannabis have been described, and variously classified as species, subspecies, or varieties:
plants cultivated for fiber and seed production, described as low-intoxicant, non-drug, or fiber types.
plants cultivated for drug production, described as high-intoxicant or drug types.
escaped, hybridised, or wild forms of either of the above types.
Cannabis plants produce a unique family of terpeno-phenolic compounds called cannabinoids, some of which produce the "high" which may be experienced from consuming marijuana. There are 483 identifiable chemical constituents known to exist in the cannabis plant, and at least 85 different cannabinoids have been isolated from the plant. The two cannabinoids usually produced in greatest abundance are cannabidiol (CBD) and/or Δ⁹-tetrahydrocannabinol (THC), but only THC is psychoactive. Since the early 1970s, Cannabis plants have been categorized by their chemical phenotype or "chemotype", based on the overall amount of THC produced, and on the ratio of THC to CBD. Although overall cannabinoid production is influenced by environmental factors, the THC/CBD ratio is genetically determined and remains fixed throughout the life of a plant. Non-drug plants produce relatively low levels of THC and high levels of CBD, while drug plants produce high levels of THC and low levels of CBD. When plants of these two chemotypes cross-pollinate, the plants in the first filial (F₁) generation have an intermediate chemotype and produce intermediate amounts of CBD and THC. Female plants of this chemotype may produce enough THC to be utilized for drug production.
Whether the drug and non-drug, cultivated and wild types of Cannabis constitute a single, highly variable species, or the genus is polytypic with more than one species, has been a subject of debate for well over two centuries. This is a contentious issue because there is no universally accepted definition of a species. One widely applied criterion for species recognition is that species are "groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups." Populations that are physiologically capable of interbreeding, but morphologically or genetically divergent and isolated by geography or ecology, are sometimes considered to be separate species. Physiological barriers to reproduction are not known to occur within Cannabis, and plants from widely divergent sources are interfertile. However, physical barriers to gene exchange (such as the Himalayan mountain range) might have enabled Cannabis gene pools to diverge before the onset of human intervention, resulting in speciation. 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.
Cannabis