When speaking about cannabis and psychoactivity, a discusson of CB1 receptors is imperative, as they are concentrated in the brain and the central nervous system. The differing effect of CBD and THC, however, boils down to their interaction with the cannabinoid 1 receptor (CB1). While THC binds very well with CB1 receptors, CBD has low binding affinity for CB1 receptors.
Due to the fact that CBD is not a good fit with CB1 receptors, it does not act directly to activate or suppress CB1 receptors, but suppresses the CB1-activating qualities of a cannabinoid like THC. In other words, CBD slightly counteracts THC’s directly stimulation of CB1 receptors, reducing the psychoactive effects of THC.