The reason behind the different effects of THC and THCA lies in their molecular structures. THC molecules are able to fit into CB1 cannabinoid receptors, which are responsible for the psychoactive effects of cannabis. However, THCA molecules have different shapes and sizes, and they are unable to bind effectively with CB1 receptors. This is why THCA is considered non-psychoactive and does not cause the 'elevated' or intoxicating effects associated with THC. Although the cannabis plant produces many cannabinoids, only a few are responsible for the psychoactive effects, with THC being the most prominent one.