The psychotropicity of cannabis, so the high sensation associated with mood changes and perception alterations from surrounding stimuli, is mainly linked to the well-known Δ9-tetrahydrocannabinol (THC-9): certain chemovars have been produced in order to higher the level of this powerful psychoactive molecule, both for medicinal and recreational purposes. Many valuable compounds accumulate in the resin of cannabis female inflorescences including terpenes, flavonoids and among others cannabinoids, cannabidiol (CBD), the major non-psychotropic constituents of the plant with potential medicinal value and capable of modulating and antagonizing THC negative physiological effects.  Within cannabinoids, also the isomers of THC-9 are present in cannabis resin: these compounds have similar molecular formula to their psychotropic counterpart, but the small differences in the structure lead to changes in the bioactivity of the molecules. Δ8-tetrahydrocannabinol (THC-8) is among the most interesting THC isomers, bearing similar pharmacological properties but milder psychoactivity and improved chemical stability: THC-8 presents lower binding affinity with cannabinoid receptors CB1 and CB2 and presents 2:3 of the potency compared to an equal dose of THC-9.  Usually THC-8 exists naturally only in small quantities (around 0.1%) and its presence is not usually significant for the overall psychotropic affect of cannabis plants: concentrated amounts of THC-8 are typically produced only through chemical reactions out of hemp-derived cannabidiol (CBD).
Breeding of different cannabis varieties for low THC levels has been a main target in hemp cultivation: some chemovars with high CBD content and levels below 0.2% THC have been grown in order to benefit from the plant-derived therapeutic effects, without the unwanted psychotropic THC action. Depending on the reason of cannabis consumption and the particular medical condition, it is possible to choose within many different cannabis chemovars with variable THC/CBD ratios or cannabis-derived products including only one or few cannabis constituents. Nevertheless it is always recommended to start from low doses and increasing gradually the amount taken, in order to monitor the personal body sensitivity to the active constituents.
References: Niesink R. J. M. et al. Does Cannabidiol Protect Against Adverse Psychological Effects of THC?. Frontiers in Psychiatry, (2013); 4(), –.doi:10.3389/fpsyt.2013.00130  Hollister L. E. et al. Delta-8- and delta-9-tetrahydrocannabinol; Comparison in man by oral and intravenous administration. Clinical Pharmacology & Therapeutics, (1973). 14(3), 353–357. doi:10.1002/cpt1973143353 [Journal impact factor = 7.266] [Times cited =79]